The present invention relates to photothermographic materials exhibiting enhanced sensitivity and superior storage stability and in particular to black-and-white photothermographic materials exhibiting enhanced sensitivity and superior storage stability of silver images after thermal development.
In the field of medical treatment and graphic arts, there have been problems in working property with respect to effluents produced from wet-processing of image forming materials, and recently, reduction of the processing effluent is strongly demanded in terms of environmental protection and space saving.
Accordingly, there are needed techniques regarding photothermographic materials for photographic use which are capable of forming black images exhibiting high sharpness, enabling efficient exposure by means of a laser imager or laser image setter. As one such a technique is known a thermally developable photosensitive material, which comprises a support having thereon an organic silver salt, light-sensitive silver halide grains and a reducing agent, as described in U.S. Pat. Nos. 3,152,904 and 3,487,075; and D. H. Klosterboer, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d (Imaging Processes and Materials, Neblette, 8th Edition, edited by J. M. Sturge, V. Walworth, and A. Shepp, page 279, 1989), etc.
One feature of the photothermographic material, which comprises a light sensitive layer containing silver halide grains as a photo-sensor and an organic silver salt as a silver ion source and an incorporated reducing agent is that images are formed through thermal development at a temperature of 80 to 140xc2x0 C., without further fixing. To achieve not only a consistent supply of silver ions to the silver halide but also prevention of deterioration in transparency caused by light scattering, it has been attempted to improve the shape of organic silver salt particles so that the particles are optimally arranged in the light sensitive layer without disadvantageously affecting light scattering.
However, in the photothermographic materials, image problems have still tended to occur because normal image formation is locally hindered due to the presence of silver halide and organic silver coagula.
On the other hand, when silver halides or organic silver salts are forcibly disperse-pulverized by employing a homogenizer, problems occur in which fog increases, sensitivity decreases, leading to deteriorated image quality. Therefore, techniques have been demanded which are capable of yielding greater light sensitivity as well as higher density without an increase in the silver amount, while decreasing fogging.
Accordingly, it is an object of the present invention to provide a thermally developable photothermographic material exhibiting enhanced sensitivity and reduced fogging caused after storage over a long period of time and also exhibiting superior stability silver images after thermal development, and an image forming process by the use thereof.
The object of the invention can be accomplished by the following constitution:
1) A photothermographic material comprising on a support a light sensitive layer containing an organic silver salt, light sensitive silver halide grains and a solvent, wherein the organic silver salt comprises tabular organic silver salt grains exhibiting an average needle ratio of not less than 1.1 and less than 10.0, the silver halide grains having been subjected to chemical sensitization
2) a photothermographic material comprising a light sensitive layer containing an organic silver salt, a light sensitive silver halide and a solvent, wherein the organic silver salt comprises tabular grains exhibiting an aspect ratio of not less than 3 and an average needle ratio measured from the major face direction of not less than 1.1 and less than 10.0; the silver halide being chemically sensitized;
3) a photothermographic material comprising a support having thereon a light sensitive layer containing an organic silver salt, a light sensitive silver halide, a reducing agent and a binder , wherein when a cross section vertical to the surface of the support is electron-microscopically observed, not less than 70% of the total projected area of the organic silver salt particles is accounted for by organic silver salt particles having a particle projected area of less than 0.025 xcexcm2 and not more than 10% of the total projected area of the organic silver salt particles is accounted for by organic silver salt particles having a particle projected area of not less than 0.2 xcexcm2; the silver halide being chemically sensitized;
4) The photothermographic material described in 1), 2) or 3), wherein the light sensitive silver halide is subjected to chemical sensitization which is performed using an organic sensitizer containing a chalcogen atom in the absence of an oxidizing agent during the process of manufacturing the photothermographic material, the silver halide being mixed with the organic silver salt, dispersed, dewatered and dried.
One of the preferred embodiments of the invention is that the light sensitive silver halide relating to this invention is subjected to chemical sensitization, which is achieved using an organic sensitizer containing a chalcogen atom in the absence of an oxidizing agent in the manufacturing process of the photothermographic material.
The organic sensitizers containing a chalcogen atom include, for example, organic sensitizers having various structures, described in JP-A No 60-150046, 4-109240 and 11-218874 (hereinafter, the term, JP-A refers to an unexamined, published Japanese Patent Application).
Examples of atomic groups containing a group for promoting adsorption onto silver halide include an atomic group containing a mercapto group (e.g., mercaptooxadiazole, mercaptotetrazole, mercaptotriazole, mercaptodiazole, mercaptothiazole, mercaptothiadiazole, mercaptooxazole, mercaptoimidazole, mercaptobenzothiazole, mercaptobenzoxazole, mercaptobenzimidazole, mercaptotetrazaindene, mercaptopyridyl, mercaptoquinolyl, 2-mercaptopyridyl, mercaptophenyl, mercaptonaphthyl, etc.), an atomic group containing a thione group (e.g., thiazoline-2-thione, oxazoline-2-thione, imidazoline-2-thione, benzothiazoline-2-thione, benzimidazoline-2-thione, thiazolidine-2-thion, etc.), an atomic group capable of forming an imino silver (e.g., triazole, tetrazole, benzotriazole, hydroxyazaindene, benzimidazole, indazole, etc.), and an atomic group containing an ethenyl group (e.g., 2-[N-(2-propynyl)amino]benzothiazole, N-(propynyl)carbazole, etc.}.
In this invention, the group for promoting adsorption onto silver halide is distinct from a labile chalcogen atom site. Thus, it is not a group capable of releasing a chalcogen atom to form a silver chalcogenide.
In this invention, the compound containing a labile chalcogen atom site refers to a compound capable of forming a silver chalcogenide in the presence of silver nitrate. Further, the chalcogen atom means a sulfur atom, selenium atom or tellurium atom.
Examples of the atomic group containing a labile sulfur atom site include a thiourea group containing atominc group [e.g., N,Nxe2x80x2-diethylthiourea, N-ethyl-Nxe2x80x2-(2-thiazolyl)thiourea, N,N-dimethylthiourea, N-phenylthiourea, etc.], a thioamido group containing atomic group (e.g., thiobenzamide, thioacetoamide, etc.), an atomic group containing a polysulfide or phosphine sulfide group [e.g., bis(pentafluorophenyl)phenylphoshine sulfide, diethylphosphine sulfide, dimethylphenylphosphine sulfide, etc.], and a thioxoazolidinone group containing atomic group (e.g., ethylrhodanine, 5-benzylidene-3-ethylrhodanine, 1,3-diphenyl-2-thiohydantoin, 3-ethyl-4-oxooxazolidine-2-thione, etc.).
Examples of the atomic group containing a labile selenium atom site include a selenourea group containing atomic group [e.g., N,N-dimethylselenourea, selenourea, N-acetyl-N,Nxe2x80x2-diethylselenourea, N-trifluoroacetyl-Nxe2x80x2,Nxe2x80x2-dimethylselenourea, N-ethyl-Nxe2x80x2-(2-thiazolyl)selenourea, N,Nxe2x80x2-diphenylselenourea, etc.], a selenoamido group containing atmic group (e.g., N-methyl-selenobenzamide, N-phenyl-selenobenzamide, N-ethyl-selenobenzamide, etc.), a phosphine selenide group containing atomic group (e.g., triphenylphosphine selenide, diphenyl[pentafluorophenyl]phosphine selenide, trism-chlorophenyl)phosphine selenide, etc.], a selenophosphate group containing atomic group [tris(p-tolyl)selenophosphate, etc.], a seleno-ester group containing atomic group [e.g., p-methoxyselenobenzoic acid (xe2x95x90O) isopropyl ester, selenobenzoic acid (xe2x95x90Se)-3xe2x80x2-oxobutyl ester, p-methoxyselenobenzoic acid (xe2x95x90Se)-3xe2x80x2-oxocyclohexyl ester, etc.], a selenide grou containing atomic group [e.g., bis(2,6-dimethoxybenzoyl)selenide, bis(n-butoxycarbonyl)selenide, bis(benzyloxycarbonyl)selenide, bis(N,N-dimethylcarbamoyl)selenide, etc.], triselenane group containing atomic group [2,4,6-tris(p-methoxyphenyl)triselenane, etc.], and a seleno ketone group containing atomic group (e.g., 4-methoxyselenoacetophenone, 4,4-methoxyselenobenzophenone, etc.).
Examples of the atomic group containing a labile tellurium atom site include a phosphine telluride group containing atomic group (e.g., butyl-di-isopropylphosphine telluride, triscyclohexylphosphine telluride, etc.), a tellurourea group containing atomic group (e.g., N,Nxe2x80x2-diethyl-N,Nxe2x80x2-diethylenetellurourea, N,Nxe2x80x2-dimethylene-N,Nxe2x80x2-dimethyltellurourea, etc.), a telluroamido group containing atomic group [e.g., N,N-dimetyl-telluroamide, N,N-tetramethylene-(p-tolyl)tellurophosphate, tributyltellurophosphate, etc.], and a tellurophosphoric amido group containing atomic group (e.g., hexamethyltellurophosphoric amide, etc.).
In addition to the foregoing, atomic groups containing a labile selenium or tellurium site may be selected from the compounds described in JP-A Nos. 4-25832, 4-109240, 4-147250, 4-33043, 5-40324, 5-24332, 5-24333, 5-303157, 5-306268, 6-27573, 6-43576, 6-75328, 6-17528, 6-180478, 6-17529, 6-208184, 6-208186, 6-317867, 7-92599, 7-98483, 7-104415, 7-140579, and 7-301880.
The compound containing a group for promoting adsorption onto silver halide and a labile chalcogen atom site within the molecule is preferably a compound represented by the following formula(C): 
wherein A1 represents an atomic group containing a group for promoting adsorption onto silver halide; L1 represents a bivalent linkage group; Z1 represents an atomic group containing a labile chalcogen atom site; W1, W2 and W3 each represent a carboxylic acid group, a sulfonic acid group, sulfinic acid group, phosphoric acid group, phosphorous acid group or a boric acid group; m1 is 0 or 1; n1 is an integer of 1 to 3; 11, 12, and 13 each are an integer of 0 to 2.
The bivalent linkage group represented by L1 is a group comprised of a carbon atom, hydrogen atom, oxygen atom, nitrogen atom or a sulfur atom; exemplary examples thereof include an alkylene group having 1 to 20 carbon atoms (e.g., methylene, ethylene, propylene, hexylene), an arylene group (e.g., phenylene, naphthylene), xe2x80x94CONR1xe2x80x94, xe2x80x94SO2NR2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NR3xe2x80x94, xe2x80x94NR4COxe2x80x94, xe2x80x94NR5SO2xe2x80x94, xe2x80x94NR6CONR7xe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94 and a combination of these groups, in which R1, R2, R3, R4, R5, R6 and R7 each represents a hydrogen atom, an aliphatic group, alicyclic group, an aromatic group or a heterocyclic group. The aliphatic group represented by R1 through R7 include an alkyl group (e.g., methyl, ethyl, isopropyl, 2-ethyl-hexyl, etc.9, an alkenyl group (e.g., propenyl, 3-pentenyl, 2-butenyl, cyclohexynyl, etc.), an alkynyl group (e.g., propargyl, 3-pentynyl, etc.), and an aralkyl group (e.g., benzyl, phenethyl, etc.). The alicyclic groups include an alicyclic groups having 6 to 10 carbon atoms (e.g., cyclopentyl, cyclohexyl, etc.); the aromatic groups include a monocyclic groups having 5 to 8 carbon atoms and its condensed ring groups (e.g., phenyl, naphthyl, etc.); the heterocyclic groups include oxygen, sulfur or nitrogen containing, 5- to 7-membered monocyclic groups and those which are condensed with another ring, such as furyl, thienyl, benzofuryl, pyrrolyl, indolyl, thiazolyl, imidazolyl, morpholyl, piperadyl, and pyrazyl. The groups represented by R1 through R7 each may be substituted by any atom or group at any position. Examples of substituent atom or group include hydroxy group, a halogen atom (e.g., fluorine atom, chlorine atom, bromine atom, iodine atom), cyano group, an amino group (e.g., methylamino, anilino, diethylamino, 2-hydroxyethylamino), an acyl group (e.g., acetyl, benzoyl, propanoyl), a carbamoyl group e.g., carbamoyl, N-methylcarbamoyl, N,N-tetramethylenecarbamoyl, N-methanesulfonylcarbamoyl, N-acetylcarbamoyl), an alkoxy group (e.g., methoxy, ethoxy, 2-hydroxyethoxy, 2-methoxyethoxy), an alkoxycarbonyl group (e.g., methoxycarbonyl, ethoxycarbonyl, 2-ethoxycarbonyl, 2-methoxyethoxycarbonyl), a sulfonyl group (e.g., methanesulfonyl, trifluoromethanesulfonyl, benzenesulfonyl, p-toluenesulfonyl), a sulfamoyl group (e.g., e.g., sulfamoyl, N,N-dimethylsulfamoyl, morpholinosulfonyl, N-ethylsulfamoyl), an acylamino group (e.g., acetoamide, trifluoroacetoamide, benzamido, thienocarbonylamino, benzenesulfonamido, and an alkoxycarbonylamino group (methoxycarbonylamino, N-methyl-ethoxycarbonylamino).
The carboxylic acid group, a sulfonic acid group, sulfinic acid group, phosphoric acid group, phosphorous acid group or a boric acid group represented by W1, W2 and W3 may be in the form of a free acid or may form a counter salt with an alkali metal, an alkaline earth metal, ammonium or an organic amine.
Examples of the compounds represented by formula (C) are shown below but are by no means limited to these. 
The amount of the chalcogen compound (i.e., chalcogen atom-containing compound) to be used is variable, depending of the kind thereof, silver halide grains and chemical. sensitization environment but is preferably 10xe2x88x928 to 10xe2x88x922 and more preferably 10xe2x88x927 to 10xe2x88x923 mol per mol of silver halide. The chemical sensitization environment relating to this invention is not specifically limited, but the pAg is preferably 6 to 11, and more preferably 7 to 10; the pH is preferably 4 to 10, and more preferably 5 to 8, and the temperature is preferably 40 to 90xc2x0 C., and more preferably 45 to 80xc2x0 C.
In one preferred embodiment of chemical sensitization relating to this invention, light sensitive silver halide is subjected to chemical sensitization, which is achieved using an organic sensitizer containing a chalcogen atom, substantially in the absence of an oxidizing agent during the process of manufacturing the photothermographic material. Herein, the expression xe2x80x9csubstantially in the absence of an oxidizing agentxe2x80x9d means that the oxidizing agent is present in an amount exerting no effect on photographic performance or no oxidizing agent is present. This chemical sensitization is different from chemical sensitization which is performed in the presence of an oxidizing agent such as pyridinium perbromide hydrobromide (or pyridinium bromide perbromide), as disclosed in U.S. Pat. No. 5,891,615. Thus, the chemical sensitization relating to this invention is performed using an organic sensitizer containing a chalcogen atom under conditions in which an oxidizing agent is not concurrently present and in the subsequent process, an oxidizing agent may be added thereto for purposes other than sensitization, such as antifogging, in which addition of the oxidizing agent is controlled so that chemical sensitization is not additionally performed. The oxidizing agent relating to this invention refers to a compound capable of oxidizing a cluster comprised of one or more silver atoms or a silver chalcogenide. Examples of the compound having such a function include a nitrogen-containing heterocyclic compound hydrogen bromide salt, such as piridinium perbromide hydrobromide, as described in U.S. Pat. No. 5,028,523, European Patent No. 460,826, and JP-A No. 4-232939; a halogen compound, such as 2-(tribromomethylsulfonyl)quinoline, as described in JP-A Nos. 50-12028 and 6-208193; and a compound capable of releasing a halogen atom, as described later.
To add the oxidizing agent under such controlled conditions that the addition does not substantially cause any additional chemical sensitization, it is preferable to cause 100% of the organic sensitizer to react in the process of chemical sensitization or to remove the unreacted chemical sensitizer in the process of washing, cause the unreacted organic sensitizer to decompose with an oxidizing agent having no sensitizing effect, or to remove it in the subsequent process.
Silver halide which is to be subjected to chemical sensitization may be formed in the presence of an organic silver salt, may be formed in the absence of an organic silver salt, or may be a mixture thereof.
As a reducing agent used in photothermographic materials are employed reducing agents containing a proton, such as bisphenols and sulfonamidophenols. Accordingly, a compound generating a labile species which is capable of abstracting a proton to deactivate the reducing agent is preferred. More preferred is a compound as a non-colored photooxidizing substance, which is capable of generating a free radical as a labile species on exposure. Any compound having such a function is applicable. However, a halogen radical, which easily forms silver halide is not preferred. An organic free radical composed of plural atoms is preferred. Any compound having such a function and exhibiting no adverse effect on the photothermographic material is usable irrespective of its structure.
Of such free radical generation compounds, a compound containing an aromatic, and carbocyclic or heterocyclic group is preferred, which provides stability to the generated free radical so as to be in contact with the reducing agent for a period sufficient to react with the reducing agent to deactivate it. Representative examples of such compounds include biimidazolyl compounds and iodonium compounds. The imidazolyl compounds generate two imidazolyl radicals as a free radical upon exposure to ultraviolet or visible radiation, which are capable of oxidizing a reducing agent remaining after development, thereby inhibiting reduction of silver salts. It is surprising that the imidazolyl compound is photo-active and capable of oxidizing a reducing agent effective in heat-promoted reduction of a substantially non-photosensitive organic silver salt.
Of such imidazolyl compounds, a compound represented by the following formula [1] is preferred: 
wherein R1, R2 and R3 (,which may be the same or different) each are a hydrogen atom, an alkyl group (e.g., methyl, ethyl, hexyl), an alkenyl group (e.g., vinyl, allyl), an alkoxyl group (e.g., methoxy, ethoxy, octyloxy), an aryl group (e.g., phenyl, naphthyl, tolyl), hydroxy, a hydrogen atom, a halogen atom, an aryloxyl (e.g., phenoxy), an alkylthio group (e.g., methlthio, butylthio), an arylthio group (e.g., phenylthio), a heterocyclic group (e.g., pyridyl, triazyl), an acyl group (e.g., acetyl, propionyl, butylyl, valeryl), a sulfonyl group (e.g., methylsulfonyl, phenylsulfonyl), an acylamino group, sulfonylamino group, an acyloxy group (e.g., acetoxy, benzoxy), carboxy, cyano, a sulfo group, or an amino group. Of these groups are preferred an aryl group, a heterocyclic group, an alkenyl group and cyano group.
The biimidazolyl compounds can be synthesized in accordance with the methods described in U.S. Pat. No. 3,734,733 and British Patent 1,271,177. Preferred Examples thereof are shown below.
Similarly preferred compounds include iodonium compounds represented by the following formula (2): 
wherein Q is a group of atoms necessary to complete a 5-, 6-, or 7-membered ring, and the atoms being selected from a carbon atom, nitrogen atom, oxygen atom and sulfur atom; and R1, R2 and R3 (,which may be the same or different) are each a hydrogen atom, an alkyl group (e.g., methyl, ethyl, hexyl), an alkenyl group (e.g., vinyl, allyl), an alkoxyl group (e.g., methoxy, ethoxy, octyloxy), an aryl group (e.g., phenyl, naphthyl, tolyl), hydroxy, a halogen atom, an aryloxyl (e.g., phenoxy), an alkylthio group (e.g., methylthio, butylthio), an arylthio group (e.g., phenylthio), an acyl group (e.g., acetyl, propionyl, butylyl, valeryl), a sulfonyl group (e.g., methylsulfonyl, phenylsulfonyl), an acylamino group, sulfonylamino group, an acyloxy group (e.g., acetoxy, benzoxy), carboxy, cyano, a sulfo group, or an amino group. Of these groups are preferred an aryl group, an alkenyl group and cyano group, provided that R1, R2 and R3 may be bonded with each other to form a ring; R4 is a carboxylate group such as acetate, benzoate or trifluoroacetate, or Oxe2x88x92; W is 0 or 1, provided that when R3 is a sulfo group or a carboxy group, W is O and R4 is Oxe2x88x92; Xxe2x88x92 is an anionic counter ion, including CH3CO2xe2x80x94, CH3SO3xe2x80x94 and PF6xe2x88x92.
Of these is specifically preferred a compound represented by the following formula [3]: 
wherein R1, R2, R3, R4, Xxe2x88x92 and W are each the same as defined in formula [2]; Y is a carbon (i.e., xe2x80x94CHxe2x95x90) to form a benzene ring or a nitrogen atom (xe2x80x94Nxe2x95x90) to form a pyridine ring.
The iodonium compounds described above can be synthesized in accordance with the methods described in Org. Syn., 1961 and Fieser, xe2x80x9cAdvanced Organic Chemistryxe2x80x9d (Reinhold, N.Y., 1961). Examples of the suitable compounds are represented by the following general formulas.

The compound releasing a labile species other than a halogen atom, such as represented by formula [1] or [2] is incorporated preferably in an amount of 0.001 to 0.1 mol/m2, and more preferably 0.005 to 0.05 mol/m2. The compound may be incorporated into any component layer of the photothermographic material relating to the invention and is preferably incorporated in the vicinity of a reducing agent.
As a compound capable of deactivating a reducing agent to inhibit reduction of an organic silver salt to silver by the reducing agent are preferred compounds releasing a labile species other than a halogen atom. However, these compounds may be used in combination with a compound capable of releasing a halogen atom as a labile species. The compound capable of releasing a halogen atom as a labile species is used preferably in an amount of 0.001 to 0.1 mol/m2 and more preferably 0.005 to 0.05 mol/m2. Exemplary examples of the compound releasing an active halogen atom include a compound represented by the following formula [4]: 
wherein Q is an aryl group or a heterocyclic group; X1, X2 and X3 are each a hydrogen atom, a halogen atom, a haloalkyl group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a sulfonyl group, an aryl group or a heterocyclic group, provided that at least of them a halogen atom; Y is xe2x80x94C(xe2x95x90O)xe2x80x94, xe2x80x94SOxe2x80x94 or xe2x80x94SO2xe2x80x94. The aryl group represented by Q may be a monocyclic group or condensed ring group and is preferably a monocyclic or di-cyclic aryl group having 6 to 30 carbon atoms (e.g., phenyl, naphthyl), more preferably a phenyl or naphthyl group, and still more preferably a phenyl group. The heterocyclic group represented by Q is a 3- to 10-membered, saturated or unsaturated heterocyclic group containing at least one of N, O and S, which may be a monocyclic or condensed with another ring to a condensed ring.
The heterocyclic group is preferably a 5- or 6-membered unsaturated heterocyclic group, which may be condensed, more preferably a 5- or 6-membered aromatic heterocyclic group, which may be condensed, still more preferably a N-containing 5- or 6-membered aromatic heterocyclic group, which may be condensed, and optimally a 5- or 6-membered aromatic heterocyclic group containing one to four N atoms, which may be condensed. Exemplary examples of heterocyclic rings included in the heterocyclic group include imidazole, pyrazole, pyridine, pyrimidine, pyrazine, pyridazine, triazole, triazines, indole, indazole, purine, thiazole, oxadiazole, quinoline, phthalazine, naphthyridine, quinoxaline, quinazoline, cinnoline, pteridine, acrydine, phenanthroline, phenazine, tetrazole, thiazole, oxazole, benzimidazole, benzoxazole, benzthiazole, indolenine and tetrazaindene. Of these are preferred imidazole, pyridine, pyrimidine, pyrazine, pyridazine, triazole, triazines, thiadiazole, oxadiazole, quinoline, phthalazine, naphthylizine, quinoxaline, quinazoline, cinnoline, tetrazole, thiazole, oxazole, benzimidazole, and tetrazaindene; more preferably imidazole, pyrimidine, pyridine, pyrazine, pyridazine, triazole, triazines, thiadiazole, quinoline, phthalazine, naphthyridine, quinoxaline, quinazoline, cinnoline, tetrazole, thiazole, benzimidazole, and benzthiazole; and still more preferably pyridine, thiazole, quinoline and benzthiazole.
The aryl group or heterocyclic group represented by Q may be substituted by a substituent, in addition to -Y-C (X1) (X2) (X3). Preferred examples of the substituent include an alkyl group, an alkenyl group, an aryl group, an alkoxyl group, an aryloxyl group, an acyloxy group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyloxy group, an acylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, a sulfonylamino group, a sulfamoyl group, a carbamoyl group, a sulfonyl group, a ureido group, phosphoramido group, a halogen atom, cyano group, sulfo group, carboxy group, nitro group and heterocyclic group. Of these are preferred an alkyl group, an aryl group, an alkoxyl group, an aryloxyl group, an acyl group, an acylamino group, an aryloxyl group, acyl group, an acylamino group, an alkoxycarbonyl group, an aryloxycarbonylamino group, a sulfonylamino group, a sulfamoyl group, a carbamoyl group, a ureido group, phosphoramido group, a halogen atom, cyano group, nitro group, and a heterocyclic group; and more preferably an alkyl group, an aryl group, an alkoxyl group, an aryloxyl group, an acyl group, an acylamino group, a sulfonylamino group, a sulfamoyl group, a carbamoyl group, a halogen group, cyano group, nitro group and a heterocyclic group; and still more preferably an alkyl group, an aryl group and a halogen atom.
X1, X2 and X3 are preferably a halogen atom, a haloalkyl group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a carbamoyl group, a sulfamoyl group, a sulfonyl group, and a heterocyclic group, more preferably a halogen atom, a haloalkyl group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, and a sulfonyl group; and still more preferably a halogen atom and trihalomethyl group; and most preferably a halogen atom. Of halogen atoms are preferably chlorine atom, bromine and iodine atom, and more preferably chlorine atom and bromine atom, and still more preferably bromine atom. Y is xe2x80x94C(xe2x95x90O)xe2x80x94, xe2x80x94SOxe2x80x94, and xe2x80x94SO2xe2x80x94, and preferably xe2x80x94SO2xe2x80x94.
Exemplary examples of these compounds are shown below. 
These compounds are incorporated in an amount within a range such that increased formation of print-out silver produces substantially no problem, preferably in an amount of not more than 150%, and more preferably not more than 100% based on the compound releasing a labile species other than a halogen atom. As afore-mentioned, these compounds deactivate a reducing agent included in the thermally developable light sensitive layer, enhancing storage stability of the photothermographic material. Reducing agents used in the photothermographic materials and capable of deactivating a free radical will be described.
Reducing agents are incorporated into the photothermographic material of the present invention. Examples of suitable reducing agents are described in U.S. Pat. Nos. 3,770,448, 3,773,512, and 3,593,863, and Research Disclosure Items 17029 and 29963, and include the following: aminohydroxycycloalkenone compounds (for example, 2-hydroxypiperidino-2-cyclohexane); esters of amino reductones as the precursor of reducing agents (for example, piperidinohexose reducton monoacetate); N-hydroxyurea derivatives (for example, N-p-methylphenyl-N-hydroxyurea); hydrazones of aldehydes or ketones (for example, anthracenealdehyde phenylhydrazone; phosphamidophenols; phosphamidoanilines; polyhydroxybenzenes (for example, hydroquinone, t-butylhydroquinone, isopropylhydroquinone, and (2,5-dihydroxy-phenyl)methylsulfone); sulfydroxamic acids (for example, benzenesulfhydroxamic acid); sulfonamidoanilines (for example, 4-(N-methanesulfonamide)aniline); 2-tetrazolylthiohydroquinones (for example, 2-methyl-5-(l-phenyl-5-tetrazolylthio)hydroquinone); tetrahydroquionoxalines (for example, 1,2,3,4-tetrahydroquinoxaline); amidoxines; azines (for example, combinations of aliphatic carboxylic acid arylhydrazides with ascorbic acid); combinations of polyhydroxybenzenes and hydroxylamines, reductones and/or hydrazine; hydroxamic acids; combinations of azines with sulfonamidophenols; xcex1-cyanophenylacetic acid derivatives; combinations of bis-xcex2-naphthol with 1,3-dihydroxybenzene derivatives; 5-pyrazolones, sulfonamidophenol reducing agents, 2-phenylindane-1,3-dione, etc.; chroman; 1,4-dihydropyridines (for example, 2,6-dimethoxy-3,5-dicarboethoxy-1,4-dihydropyridine); bisphenols (for example, bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane, bis(6-hydroxy-m-tri)mesitol, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4,5-ethylidene-bis(2-t-butyl-6-methyl)phenol, UV-sensitive ascorbic acid derivatives and 3-pyrazolidones. Of these, particularly preferred reducing agents are hindered phenols. As preferred hindered phenols, listed are compounds represented by the general formula (A) described below: 
wherein R represents a hydrogen atom or an alkyl group having from 1 to 10 carbon atoms (for example, isopropyl, xe2x80x94C4H9, 2,4,4-trimethylpentyl), and Rxe2x80x2 and Rxe2x80x3 each represents an alkyl group having from 1 to 5 carbon atoms (for example, methyl, ethyl, t-butyl).
Exemplary examples of the compounds represented by the formula (A) are shown below. 
The used amount of reducing agents represented by the above-mentioned general formula (A) is preferably between 1xc3x9710xe2x88x922 and 10 moles, and is more preferably between 1xc3x9710xe2x88x922 and 1.5 moles per mole of silver.
The photothermographic material according to the invention contains a light sensitive silver halide and an organic silver salt.
Light sensitive silver halide grains used in the invention will be described. Silver halide emulsions used in the invention can be prepared according to the methods described in P. Glafkides, Chimie Physique Photographique (published by Paul Montel Corp., 19679; G. F. Duffin, Photographic Emulsion Chemistry (published by Focal Press, 1966); V. L. Zelikman et al., Making and Coating of Photographic Emulsion (published by Focal Press, 1964). Any one of acidic precipitation, neutral precipitation and ammoniacal precipitation is applicable and the reaction mode of aqueous soluble silver salt and halide salt includes single jet addition, double jet addition and a combination thereof. Specifically, preparation of silver halide grains with controlling the grain formation condition, so-called controlled double-jet precipitation is preferred. The halide composition of silver halide is not specifically limited and may be any one of silver chloride, silver chlorobromide, silver iodochlorobromide, silver bromide, silver iodobromide and silver iodide.
The grain formation process is usually classified into two stages of formation of silver halide seed crystal grains (nucleation) and grain growth. These stages may continuously be conducted, or nucleation (seed grain formation) and grain growth may be separately performed. The controlled double-jet precipitation, in which grain formation is undergone with controlling grain forming conditions such as pAg and pH, is preferred to control the grain form or grain size. In cases when nucleation and grain growth are separately conducted, for example, a soluble silver salt and a soluble halide salt are homogeneously and promptly mixed in an aqueous gelatin solution to form nucleus grains (seed grains), thereafter, grain growth is performed by supplying soluble silver and halide salts, while being controlled at a pAg and pH to prepare silver halide grains. After completing the grain formation, the resulting silver halide grain emulsion is subjected to desalting to remove soluble salts by commonly known washing methods such as a noodle washing method, a flocculation method, a ultrafiltration method, or electrodialysis to obtain desired emulsion grains.
In order to minimize cloudiness after image formation and to obtain excellent image quality, the less the average grain size, the more preferred, and the average grain size is preferably not more than 0.2 xcexcm, more preferably between 0.01 and 0.17 xcexcm, and still more preferably between 0.02 and 0.14 xcexcm. The average grain size as described herein is defined as an average edge length of silver halide grains, in cases where they are so-called regular crystals in the form of cube or octahedron. Furthermore, in cases where grains are tabular grains, the grain size refers to the diameter of a circle having the same area as the projected area of the major faces. Furthermore, silver halide grains are preferably monodisperse grains. The monodisperse grains as described herein refer to grains having a coefficient of variation of grain size obtained by the formula described below of not more than 7%; more preferably not more than 5%, still more preferably not more than 3%, and most preferably not more than 1%.
Coefficient of variation of grain size (%)=standard deviation of grain diameter/average grain diameterxc3x97100
The grain form includes cubic, octahedral or tetradecahedral grains, tabular grains, spherical grains, bar-like grains, and potato-shaped grains. Of these, cubic grains, octahedral grains, tetradecahedral grains and tabular grains are specifically preferred.
The aspect ratio of tabular grains is preferably 2 to 100, and more preferably 3 to 50. These grains are described in U.S. Pat. Nos. 5,264,337, 5,314,798 and 5,320,958 and desired tabular grains can be readily obtained. Silver halide grains having rounded corners are also preferably employed.
The silver halide grain shape is not specifically limited, but a high ratio accounted for by a Miller index [100] plane is preferred. This ratio is preferably at least 50%; is more preferably at least 70%, and is most preferably at least 80%. The ratio accounted for by the Miller index [100] face can be obtained based on T. Tani, J. Imaging Sci., 29, 165 (1985) in which adsorption dependency of a [111] face or a [100] face is utilized.
It is preferred to use low molecular gelatin having an average molecular weight of not more than 50,000 in the preparation of silver halide grains used in the invention, specifically, in the stage of nucleation. Thus, the low molecular gelatin has an average molecular eight of not more than 50,000, preferably 2,000 to 40,000, and more preferably 5,000 to 25,000. The average molecular weight can be determined by means of gel permeation chromatography. The low molecular gelatin can be obtained by subjecting an aqueous gelatin conventionally used and having an average molecular weight of ca. 100,000 to enzymatic hydrolysis, acid or alkali hydrolysis, thermal degradation at atmospheric pressure or under high pressure or ultrasonic degradation.
The concentration of dispersion medium used in the nucleation stage is preferably not more than 5% by weight, and more preferably 0.05 to 3.0% by weight.
In the preparation of silver halide grains, it is preferred to use a compound represent by the following formula [5], specifically in the nucleation stage:
Formula [5]
YO(CH2CH2O)m(C(CH3)HCH2O)p(CH2CH2O)nY 
wherein Y is a hydrogen atom, xe2x80x94SO3M or xe2x80x94CO-B-COOM, in which M is a hydrogen atom, alkali metal atom, ammonium group or ammonium group substituted by an alkyl group having carbon atoms of not more than 5, and B is a chained or cyclic group forming an organic dibasic acid; m and n each are 0 to 50; and p is 1 to 100. The compound represented by formula [5] has been employed as a defoaming agent to inhibit marked foaming occurred when stirring or moving emulsion raw materials, specifically in the stage of preparing an aqueous gelatin solution, adding a water-soluble silver and halide salts to the aqueous gelatin solution or coating an emulsion on a support during the process of preparing silver halide photographic light sensitive materials. A technique of using these compounds as a defoaming agent is described in JP-A 44-9497. The compound represented by formula [5] also functions as a defoaming agent during nucleation.
The compound represented by formula [5] is used preferably in an amount of not more than 1%, and more preferably 0.01 to 0.1% by weight, based on silver. The compound is to be present at the stage of nucleation, and may be added to a dispersing medium prior to or during nucleation. Alternatively, the compound may be added to an aqueous silver salt solution or halide solution used for nucleation. It is preferred to add it to a halide solution or both silver salt and halide solutions in an amount of 0.01 to 2.0% by weight. It is also preferred to make the compound represented by formula [5] present over a period of at least 50% (more preferably, at least 70%) of the nucleation stage. The compound may be added in the form of powder or solution using a solvent such as methanol. Representative examples of the compound represented by formula [5] are shown below, but are not limited to these.
E-1
HO(CH2CH2O)m(C(CH3)HCH2O)19.8(CH2CH2O)nH (m+n=9.77)
E-2
NaO2C(CH2)OCO(CH2CH2O)m(C(CH3)HCH2O)17(CH2CH2O)nCO(CH2)2CO2Na (m+n=5.7)
E-3
KO2CCHxe2x95x90CHCOO(CH2CH2O)m(C(CH3)HCH2O)34.2(CH2CH2O)nCOCHxe2x95x90CHCO2K (m+n=8.5)
E-4
NaO3SO(C(CH3)HCH2O)17SO3Na
E-4

The temperature during the stage of nucleation is preferably 5 to 60xc2x0 C., and more preferably 15 to 50xc2x0 C. Even when nucleation is conducted at a constant temperature, in a temperature-increasing pattern (e.g., in such a manner that nucleation starts at 25xc2x0 C. and the temperature is gradually increased to reach 40xc2x0 C. at the time of completion of nucleation) or its reverse pattern, it is preferred to control the temperature within the range described above.
Silver salt and halide salt solutions used for nucleation are preferably in a concentration of not more than 3.5N, and more preferably 0.01 to 2.5N. The flow rate of aqueous silver salt solution is preferably 1.5xc3x9710xe2x88x923 to 3.0xc3x9710xe2x88x921 /min per lit. of the solution, and more preferably 3.0xc3x9710xe2x88x923 to 8.0xc3x9710xe2x88x922 mol/min. per lit. of the solution. The pH during nucleation is within a range of 1.7 to 10, and since the pH at the alkaline side broadens the grain size distribution, the pH is preferably 2 to 6. The pBr during nucleation is 0.05 to 3.0, preferably 1.0 to 2.5, and more preferably 1.5 to 2.0.
Silver halide may be incorporated into an image forming layer by any means, in which silver halide is arranged so as to be as close to reducible silver source as possible. It is general that silver halide, which has been prepared in advance, added to a solution used for preparing an organic silver salt. In this case, preparation of silver halide and that of an organic silver salt are separately performed, making it easier to control the preparation thereof. Alternatively, as described in British Patent 1,447,454, silver halide and an organic silver salt can be simultaneously formed by allowing a halide component to be present together with an organic silver salt-forming component and by introducing silver ions thereto.
Silver halide can also be prepared by reacting a halogen containing compound with an organic silver salt through conversion of the organic silver salt. Thus, a silver halide-forming component is allowed to act onto a pre-formed organic silver salt solution or dispersion or a sheet material containing an organic silver salt to convert a part of the organic silver salt to light sensitive silver halide. The thus formed silver halide is effectively in contact with the organic silver salt, exhibiting favorable actions. In this case, the silver halide-forming component refers to a compound capable of forming silver salt upon reaction with the organic silver salt. Such a compound can be distinguished by the following simple test. Thus, a compound to be tested is to be mixed with the organic silver salt, and if necessary, the presence of a specific peak in silver halide can be confirmed by the X-ray diffractometry, after heating. Compounds that have been confirmed to be effective as a silver halide-forming component include inorganic halide compounds, onium halides, halogenated hydrocarbons, N-halogen compounds and other halogen containing compounds. These compounds are detailed in U.S. Pat. Nos. 4,009,039, 3,457,075 and 4,003,749, British Patent 1,498,956 and JP-A 53-27027 and 53-25420. Exemplary examples thereof are shown below:
(1) Inorganic halide compound: e.g., a halide compound represented by formula, Mxc3x97n, in which M represents H, NH4 or a metal atom; n is 1 when M is H or NH4 and a number equivalent to a valence number of the metal atom when M is the metal atom; the metal atom includes lithium, sodium, potassium, cesium, magnesium, calcium, strontium, barium, zinc, cadmium, mercury, tin, antimony, chromium, manganese, cobalt, rhodium, and cerium, and molecular halogen such as aqueous bromine being also effective;
(2) Onium halide: e.g., quaternary ammonium halides such as trimethylphenylammonium bromide, cetylethyldimethylammonium bromide, and trimethylbenzylammonium bromide; and tertiary sulfonium halides such as trimethylsulfonium iodide;
(3) Halogenated hydrocarbons: e.g., iodoform, bromoform, carbon tetrachloride and 2-brom-2-methylpropane;
(4) N-halogeno compounds: e.g., N-chlorosuccinimide, N-bromosucciimde, N-bromophthalimide, N-bromoacetoamide, N-iodosuccinimide, N-bromophthalazinone, N-bromooxazolinone, N-chlorophthalazinone, N-bromoacetoanilide, N,N-dibromobenzenesulfonamide, N-bromo-N-methylbenzenesulfonamide, 1,3-dibromo-4,4-dimethylhydantoin and N-bromourazole;
(5) Other halogen containing compounds: e.g., triphenylmethyl chloride, triphenylmethyl bromide 2-bromoacetic acid, 2-bromoethanol and dichlorobenzophenone.
The silver halide forming components may be used in combination. As described above, although silver halide can be formed by converting a part or all of an organic silver salt to silver halide through reaction of the organic silver salt and a halide ion, it is preferred to use silver halide separately prepared which can be easily controlled with respect to grain size or grain form. The silver halide separately prepared may be used in combination with silver halide prepared by conversion of at least apart of an organic silver salt. The silver halide which is separately prepared or prepared through conversion of an organic silver salt is used preferably in an amount of 0.001 to 0.7 mol, and more preferably 0.03 to 0.5 mol per mol of organic silver salt.
Silver halide preferably occludes ions of metals belonging to Groups 6 to 11 of the Periodic Table. Preferred as the metals are W; Fe, Co, Ni, Cu, Ru, Rh, Pd, Re, Os, Ir, Pt and Au. These metals may be introduced into silver halide in the form of a complex. In the present invention, regarding the transition metal complexes, six-coordinate complexes represented by the general formula described below are preferred:
Formula:(ML6)m: 
wherein M represents a transition metal selected from elements in Groups 6 to 11 of the Periodic Table; L represents a coordinating ligand; and m represents 0, 1-, 2-, 3- or 4-.
Exemplary examples of the ligand represented by L include halides (fluoride, chloride, bromide, and iodide), cyanide, cyanato, thiocyanato, selenocyanato, tellurocyanato, azido and aquo, nitrosyl, thionitrosyl, etc., of which aquo, nitrosyl and thionitrosyl are preferred. When the aquo ligand is present, one or two ligands are preferably coordinated. L may be the same or different.
Exemplary examples of transition metal ligand complexes are shown below.
1: [RhCl6]3xe2x88x92
2: [RuCl6]3xe2x88x92
3: [ReCl6]3xe2x88x92
4: [RuBr6]3xe2x88x92
5: [OsCl6]3xe2x88x92
6: [CrCl6]4xe2x88x92
7: [IrCl6]4xe2x88x92
8: [IrCl6]3xe2x88x92
9: [Ru(NO)Cl5]2xe2x88x92
10: [RuBr4(H2O)]2xe2x88x92
11: [Ru(NO) (H2O)Cl4]xe2x88x92
12: [RhCl5(H2O)]2xe2x88x92
13: [Re(NO)Cl5]2xe2x88x92
14: [Re(NO)(CN)5]2xe2x88x92
15: [Re(NO)Cl(CN)4]2xe2x88x92
16: [Rh(NO)2Cl4]xe2x88x92
17: [Rh(NO) (H2O)Cl4]xe2x88x92
18: [Ru(NO)(CN)5]2xe2x88x92
19: [Fe(CN)6]3xe2x88x92
20: [Rh(NS)Cl5]2xe2x88x92
21: [Os(NO)Cl5]2xe2x88x92
22: [Cr(NO)Cl5]2xe2x88x92
23: [Re(NO)Cl5]xe2x88x92
24: [Os(NS)Cl4(TeCN)]2xe2x88x92
25: [Ru(NS)Cl5]2xe2x88x92
26: [Re(NS)Cl4(SeCN)]2xe2x88x92
27: [Os(NS)Cl(SCN)4]2xe2x88x92
28: [Ir(NO)Cl5]2xe2x88x92
With regard to cobalt or iron compounds, hexacyano cobalt or iron complexes are preferably used and exemplary examples thereof are shown below:
29: [Fe(CN)6]4xe2x88x92
30: [Fe(CN)63xe2x88x92
31: (Co(CN)6]3xe2x88x92
Compounds, which provide these metal ions or complex ions, are preferably incorporated into silver halide grains through addition during the silver halide grain formation. These may be added during any preparation stage of the silver halide grains, that is, before or after nuclei formation, growth, physical ripening, and chemical ripening. However, these are preferably added at the stage of nuclei formation, growth, and physical ripening; furthermore, are preferably added at the stage of nuclei formation and growth; and are most preferably added at the stage of nuclei formation. These compounds may be added several times by dividing the added amount. Uniform content in the interior of a silver halide grain can be carried out. As disclosed in JP-A No. 63-29603, 2-306236, 3-167545, 4-76534, 6-110146, 5-273683, the metal can be distributively occluded in the interior of the grain.
These metal compounds can be dissolved in water or a suitable organic solvent (for example, alcohols, ethers, glycols, ketones, esters, amides, etc.) and then added. Furthermore, there are methods in which, for example, an aqueous metal compound powder solution or an aqueous solution in which a metal compound is dissolved along with NaCl and KCl is added to a water-soluble silver salt solution during grain formation or to a water-soluble halide solution; when a silver salt solution and a halide solution are simultaneously added, a metal compound is added as a third solution to form silver halide grains, while simultaneously mixing three solutions; during grain formation, an aqueous solution comprising the necessary amount of a metal compound is placed in a reaction vessel; or during silver halide preparation, dissolution is carried out by the addition of other silver halide grains previously doped with metal ions or complex ions. Specifically, the preferred method is one in which an aqueous metal compound powder solution or an aqueous solution in which a metal compound is dissolved along with NaCl and KCl is added to a water-soluble halide solution. When the addition is carried out onto grain surfaces, an aqueous solution comprising the necessary amount of a metal compound can be placed in a reaction vessel immediately after grain formation, or during physical ripening or at the completion thereof or during chemical ripening.
Silver halide grain emulsions used in the invention may be desalted after the grain formation, using the methods known in the art, such as the noodle washing method and flocculation process.
The light sensitive silver halide grains used in the invention is chemically sensitized with not only the foregoing chalcogen atom-containing organic sensitizer, but also by the commonly known chemical sensitization such as noble metal sensitization and reduction sensitization. These sensitization may be employed in combination.
Examples of the compounds used in the noble metal sensitization include chloroauric acid, potassium chloroaurate, potassium aurothiocyanate, gold sulfide, gold selenide, compounds described U.S. Pat. No. 2,448,060 and British Patent No. 618,061. Examples of the compounds used in the reduction sensitization include ascorbic acid, thiourea dioxide, stannous chloride, aminoiminomethane-sulfinic acid, hydrazine derivatives, borane compounds, silane compounds and polyamine compounds. The reduction sensitization can be carried out by ripening an emulsion with keeping the pH and pAg at not less than 7 and not more than 8.3, respectively.
Silver halide used in the invention is preferably spectral-sensitized by allowing a sensitizing dye to adsorb onto the silver halide. Spectral sensitizing dues usable in the invention include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, styryl dyes, hemi-cyanine dyes, oxonol dyes and hemi-oxonol dyes, as described in JP-A 63-159841, 60-140335, 63-231437, 63-259651, 63-304242, and 63-15245; U.S. Pat. Nos. 4,639,414, 4,740,455, 4,741,966, 4,751,175 and 4,835,096. Useful sensitizing dyes used in the invention are also described in RD17643, sect. IV-A (December, 1978, page 23) and RD18431 sect. IX (August, 1978, page 437). It is specifically preferred to use sensitizing dyes exhibiting spectral sensitivity suited for spectral characteristics of light sources used in a laser imager or a scanner, as described in JP-A 9-34078, 9-54409 and 9-80679.
Useful cyanine dyes are those which contain a basic nucleus, such as a thiazoline nucleus, oxazoline nucleus, pyrroline nucleus, pyridine nucleus, oxazole nucleus, thiazole nucleus, selenazole nucleus and imidazole nucleus. Useful preferred merocyanine dyes include acidic nuclei such as a thiohydantoin nucleus, rhodanine nucleus, oxazolidinedione nucleus, thiazolinedione nucleus, barbituric acid nucleus, thiazolinone nucleus, malononirile nucleus and pyrazolone nucleus, as well as basic nuclei described above.
It is specifically preferred to use sensitizing dyes exhibiting sensitivity to the infrared region. Examples of preferred infrared sensitizing dyes used in the invention include those described in U.S. Pat. Nos. 4,536,473, 4,515,888 and 4,959,294.
Specifically, preferred sensitizing dyes are dyes represented by the following formulas (1) to (4): 
In formulas (S1) to (S4), Y1, Y2, Y11, Y21, Y22 and Y31 each are independently an oxygen atom, a sulfur atom, a selenium atom, xe2x80x94C(Ra)(Rb)xe2x80x94 group or xe2x80x94CHxe2x95x90CHxe2x80x94 group, in which Ra and Rb each are a hydrogen atom, an alkyl group (preferably having 1 to 5 carbon atoms) or a non-metallic atom group necessary to form an aliphatic spiro ring; Z1 is a non-metallic atom group necessary to form a 5- or 6-membered ring; R1, R11, R21, R22, R31 and R32 each are an aliphatic group or a non-metallic atom group necessary to form a condensed ring between R1 and W3 or between R11 and W14; Rc and Rd each are independently an unsubstituted lower alkyl group, a cycloalkyl group, an aralkyl group, an aryl group or a heterocyclic group; W1, W2, W3, W4, W11, W12, W13, W14, W21, W22, W23, W24, W31, W32, W33 and W34 each are independently a hydrogen atom, a substituent or a non-metallic atom group necessary to form a condensed ring by bonding between W1 and W2, W11 and W12, W21 and W22, W23 and W24, W31 and W32, or W33 and W34; V1 to V9, V11 to V13, V21 to V29, and V31 to V33 each are independently a hydrogen atom, a halogen atom, an amino group, an alkylthio group, an arylthio group, a lower alkyl group, a lower alkoxy group, an aryl group, an aryloxy group, a heterocyclic group or a non-metallic atom group necessary to form a 5- to 7-membered ring by bonding between V1 and V3, V2 and V4, V3 and V5, V2 and V6, V5 and V7, V6 and V8, V7 and V9, V11 and V13, V21 and V23, V22 and V24, V23 and V25, V24 and V26, V25 and V27, V26 and V28, V27 and V29, or V31 and V33; X21 and X31, provided that at least one of V1 to V9 and at least one of V11 to V13 are a group other than a hydrogen atom; X1, X11, X21 and X31 each are an ion necessary to compensate for an intramolecular charge; 11, 111, 121 and 131 each an ion necessary to compensate for an intramolecular charge; k1, k2, k31 and k32 each are 0 or 1; n21, n22, n31 and n32 each are 0, 1 or 2;, provided that n1 and n22, and n31 and n32 are not 0 at the same time; p1 and p11 are each 0 or 1; q1 and q11 each are 1 or 2, provided that the sum of p1 and q1 and the sum of p11 and q11 each are respectively not more than 2.
Of formulas (S1) and (S2), a compound represented by the following formula (S1-1) or (S2-1) is more preferred: 
wherein Y1, Y2 and Y11 each are independently an oxygen atom, a sulfur atom, a selenium atom, xe2x80x94C(Ra)(Rb)xe2x80x94 group or xe2x80x94CHxe2x95x90CHxe2x80x94 group, in which Ra and Rb each are a hydrogen atom, a lower alkyl group or an atomic group necessary to form an aliphatic spiro-ring when Ra and Rb are linked with each other; Z1 is an atomic group necessary to form a 5- or 6-membered ring; R is a hydrogen atom, a lower alkyl, a cycloalkyl group, an aralkyl group, a lower alkoxy group, an aryl group, a hydroxy group or a halogen atom; W1, W2, W3, W4, W11, W12, W13 and W14 each are independently a hydrogen atom, a substituent or a non-metallic atom group necessary to form a condensed ring by bonding between W1 and W2 or W11 and W12; R1 and R11 are each an aliphatic group or a non-metallic atom group necessary to form a condensed ring by bonding between R1 and W3 or R11 and W14; L1 to L9, and L11 to L15 each are independently a methine group; X1 and X11 each are an ion necessary to compensate for an intramolecular charge; 11 and 111 each an ion necessary to compensate for an intramolecular charge; k1 and k2 each are 0 or 1; p1 and p11 are each 0 or 1; q1 and q11 each are 1 or 2, provided that the sum of p1 and q1 and the sum of p11 and q11 each are respectively not more than 2.
Substituents will be further described. Thus, the substituent groups of the compounds represented by formulas (S1), (S2), (S1-1), (S2-1), (S3), and (S4) will be explained below.
The 5- or 6-membered condensed rings completed by an atomic group represented by Z1 include a condensed cyclohexene ring, a condensed benzene ring, a condensed thiophene ring, a condensed pyridine ring, and a condensed naphthalene ring. Exemplary examples thereof include a benzoxazole ring, tetrahydrobenzoxazole ring, naphthooxazole ring, benzonephthooxazole ring, benzothiazole ring, tetrahydrobenzothiazole ring, naphthothiazole ring, benzonaphthothiazole ring; thienothiazole ring, thianaphthenothiazole ring, pyridothiazole ring, benzoselenazole ring, tetrahydrobenzoselenazole ring, naphthoselenazole ring, benzonaphthoselenazole ring, quinoline ring, 3,3-dialkylindolenine and 3,3-dialkylpyridopyrroline. Any substituent such as one represented by W1 to W4 described later can be substituted on the ring described above.
Examples of the aliphatic group represented by R1, R11, R21, R22, R31, and R32 include a branched or straight-chained alkyl group having 1 to 10 carbon atoms (e.g., methyl, ethyl, propyl, butyl, pentyl, i-pentyl, 2-ethyl-hexyl, octyl, decyl), an alkenyl group having 3 to 10 carbon atoms (e.g., 2-propenyl, 3-butenyl, 1-methyl-3-propenyl, 3-pentenyl, 1-methyl-3-butenyl, 4-hexenyl), and an aralkyl group having 7 to 10 carbon atoms (e.g., benzyl, phenethyl). These groups may further be substituted with a substituent, including groups such as a lower alkyl group (preferably having 1 to 5 carbon atoms, e.g., methyl, ethyl, propyl), a halogen atom (e.g., fluorine atom, chlorine atom, or bromine atom), a vinyl group, an aryl group (e.g., phenyl, p-tolyl, p-bromophenyl), trifluoromethyl, an alkoxyl group (e.g., methoxy, ethoxy, methoxyethoxy), an aryloxyl group (e.g., phenoxy, p-tolyloxy), cyano, a sulfonyl group (e.g., methanesulfonyl, trifluoromethansulfonyl), p-toluenesulfonyl), an alkoxycarbonyl group (e.g., ethoxycarbonyl, butoxycarbonyl), an amino group (e.g., amino, biscarboxymethylamino), an aryl group (e.g., phenyl, carboxyphenyl), a heterocyclic group (e.g., tetrahydrofurfuryl, 2-pyrrolidinone-1-yl), an acyl group (e.g., acetyl, benzoyl), an ureido group (e.g., ureido, 3-methylureido, 3-phenylureido), a thioureido group (e.g., thioureido, 3-methylthioureido), an alkylthio group (e.g., methylthio, ethylthio), an arylthio group (e.g., phenylthio), a heterocyclic-thio group (e.g., 2-thienythio, 3-thienylthio, 2-imidazolylthio), a carbonyloxy group (e.g., acetyloxy, propanoyloxy, benzoyloxy), an acylamino group (e.g., acetylamino, benzoylamino); and hydrophilic groups, such as a sulfo group, a carboxy group, a phosphono group, a sulfate group, hydroxy, mercapto, sulfino group, a carbamoyl group (e.g., carbamoyl, n-methylcarbamoyl, N,N-tetramethylene-carbamoyl), a sulfamoyl group (e.g., sulfamoyl, N,N-3-oxapentamethylenaminosulfonyl), a sulfonamido group (e.g., methanesulfonamido, butanesulfoneamido), a sulfonylamino-carbonyl group(e.g., methanesulfonylaminocarbonyl, ethanesulfonylaminocarbonyl), an acylaminosulfonyl group (e.g., acetoamidosulfonyl, methoxyacetoamidosulfonyl), an acylaminocarbonyl group (e.g., acetoamidocarbonyl, methoxyacetoamidocarbonyl), and a sulfinylaminocarbonyl group (e.g., methasulfinylaminocarbonyl, ethanesulfinylamino-carbonyl). Examples of aliphatic groups substituted by a hydrophilic group include carboxymethyl, carboxypentyl, 3-sulfatobutyl, 3-sulfopropyl, 2-hydroxy-3-sulfopropyl, 4-sulfobutyl, 5-sulfopentyl, 3-sulfopentyl, 3-sulfinobutyl, 3-phosphonopropyl, hydroxyethyl, N-methanesulfonylcarbamoyl-methyl, 2-carboxy-2-propenyl, o-sulfobenzyl, p-sulfobenzyl and p-carboxybenzyl.
The lower alkyl group represented by R include, for example, a straight-chained or branched one having 1 to 5 carbon atoms, such as methyl, ethyl, propyl, pentyl and isopropyl. The cycloalkyl group includes, e.g., cyclopropyl, cyclobutyl and cyclopentyl. The aralkyl group includes, e.g., benzyl, phenethyl, p-methoxyphenylmethyl and o-acetylaminophenylethyl; the lower alkoxy group includes one having 1 to 4 carbon atoms, including methoxy, ethoxy, propoxy and i-propoxy; the aryl group includes substituted or unsubstituted one, such as phenyl, 2-naphthyl, 1-naphthyl, o-tolyl, o-methoxyphenyl, m-chlorophenyl, m-bromophenyl, p-tolyl and p-ethoxyphenyl. These groups may be substituted by a substituent group, such as a phenyl group, a halogen atom (e.g., fluorine atom, chlorine atom, bromine atom, iodine atom), an alkoxy group or hydroxy.
The lower alkyl group represented by Ra or Rb are the same as defined in R.
The lower alkyl group represented by Rc, and Rd includes a straight-chained or branched one having 1 to 5 carbon atoms, such as methyl, ethyl, propyl, pentyl and isopropyl. The cycloalkyl group includes, e.g., cyclopropyl, cyclobutyl and cyclopentyl. The aralkyl group includes, e.g., benzyl, phenethyl, p-methoxyphenylmethyl and o-acetylaminophenyl-ethyl; the aryl group includes substituted or unsubstituted one, such as phenyl, 2-naphthyl, 1-naphthyl, o-tolyl, o-methoxyphenyl, m-chlorophenyl, m-bromophenyl, p-tolyl and p-ethoxyphenyl; and the heterocyclic group includes substituted or unsubstituted one, such as 2-furyl, 5-methyl-2-furyl, 2-thienyl, 2-imidazolyl, 2-methyl-1-imidazolyl, 4-phenyl-2-thiazolyl, 5-hydroxy-2-benzothiazolyl, 2-pyridyl and 1-pyrrolyl. These groups, as described above, may be substituted by a substituent group, such as a phenyl group, a halogen atom, an alkoxy group or hydroxy.
Examples of the substituent groups represented by W1 to W4, W11 to W14, W21 to W24, W31 to W34, W41 to W44 and W51 to W54 include an alkyl group (e.g., methyl, ethyl, butyl, I-butyl), an aryl group (including monocyclic and polycyclic ones such as phenyl and naphthyl), a heterocyclic group (e.g., thienyl, furyl, pyridyl, carbazolyl, pyrrolyl, indolyl), a halogen atom (e.g., fluorine atom, chlorine atom, bromine atom, iodine atom), a vinyl group, trifluoromethyl, an alkoxy group (e.g., methoxy, ethoxy, methoxyethoxy), an aryloxy group (e.g., phenoxy, p-tolyloxy), a sulfonyl group (e.g., methanesulfonyl, p-toluenesulfonyl), an alkoxycarbonyl group (e.g., ethoxycarbonyl, ethoxycarbonyl), an amino group (e.g., amino, biscarboxymethylamino), an acyl group (e.g., acetyl, benzoyl), an ureido group (e.g., ureido, 3-methylureido), a thioureido group (e.g., thioureido, 3-methylthioureido), an alkylthio group (e.g., methylthio, ethylthio), an alkenyl thio group, an arylthio group (e.g., phenylthio), hydroxy and styryl.
These groups may be substituted by the same substituent groups as described in the aliphatic group represented by R1. Examples of substituted alkyl group include 2-methoxyethyl, 2-hydroxyethyl, 3-ethoxycarbonylpropyl, 2-carbamoylethyl, 2-methanesulfonylethyl, 3-methanesulfonylaminopropyl, benzyl, phenethyl, carboxymethyl, carboxymethyl, allyl, and 2-furylethyl. Examples of substituted aryl groups include p-carboxyphenyl, p-N,N-dimethylaminophenyl, p-morpholinophenyl, p-methoxyphenyl, 3,4-dimethoxyphenyl, 3,4-methylene-dioxyphenyl, 3-chlorophenyl, and p-nitrophenyl. Further, examples of substituted heterocyclic group include 5-chloro-2-pyridyl, 2-ethoxycarbonyl-2-pyridyl and 5-carbamoyl-2-pyridyl. W1 and W2, W3 and W4, W11 and W12, W13 and W14, W21 and W22, W23 and W24, W31 and W32, W33 and W34 each pair may combine to form a condensed ring, such as 5- or 6-membered saturated or unsaturated condensed carbon rings, which are further substituted by substituent groups as described in the aliphatic group.
Among the groups represented by V1 to V9, V11 to V13, V21 to V29, and V31 to V33, the halogen atom includes, e.g., a fluorine atom, chlorine atom, bromine atom and iodine atom; the amino group includes, e.g., amino, dimethylamino, diphenylamino, and methylphenylamino; the alkylthio group includes substituted and substituted ones, such as phenylthio or m-fluorphenylthio; the lower alkyl group includes straight-chained or branched one having five or less carbon atoms, such as methyl, ethyl, propyl, butyl, pentyl or isopropyl; the lower alkoxy group includes one having four or less carbon atoms, such as methoxy, ethoxy, propoxy, or iso-propoxy; the aryl group includes substituted and unsubstituted ones, such as phenyl, 2-naphthyl, 1-naphthyl, o-tolyl, o-methoxyphenyl, m-chlorophenyl, m-bromophenyl, p-tolyl, and p-ethoxy phenyl; the aryloxyl group includes substituted and unsubstituted ones, such as phenoxy, p-tolyloxy, and m-carboxyphenyloxy; and the heterocyclic group includes substituted or unsubstituted ones, such as 2-furyl, 5-methyl-2-furyl2-thienyl, 2-imidazolyl, 2-methyl-1-imidazolyl, 4-phenyl-2-thiazolyl, 5-hydroxy-2-benzothiazolyl, 2-pyridyl, and 1-pyrrolyl. These groups may further be substituted by a substituent group, such as a phenyl group, a halogen atom, alkoxy group, or hydroxy. V1 and V3, V2 and V4, V3 and V5, V4 and V6, V5 and V7, V6 and V8, V7 and V9, V11 and V13, V21 and V23, V22 and V24, V23 and V25, V24 and V26, V25 and V27, V26 and V28, V27 and V29, and V31 and V33 each pair may combine to form a 5- to 7-membered ring, such as a cyclopentene ring, cyclohexene ring, cycloheptene ring, and decalin ring, each of which may further be substituted by a lower alkyl group, lower alkoxy group or aryl group, as described in R.
The methylene group represented by L1 to L9, L11 to L15 are each a substituted or unsubstituted methylene group. Examples of the substituent group thereof include fluorine and chlorine atoms, a substituted or unsubstituted lower alkyl group(e.g., methyl, ethyl, I-propyl, benzyl), and a substituted or unsubstituted alkoxy group (e.g., methoxy, ethoxy), a substituted or unsubstituted aryloxy group (e.g., phenoxy, naphthoxy), a substituted or unsubstituted aryl group (e.g., phenyl, naphthyl, p-tolyl, o-carboxyphenyl), N(U1) (U2), -SRg, a substituted or unsubstituted heterocyclic group [e.g., 2-thienyl, 2-furyl, N,Nxe2x80x2-bis(methoxyethyl)-barbituric acid], in which Rg is a lower alkyl group (preferably having 1 to 5 carbon atoms), an aryl group or a heterocyclic group and examples of -SRg include methylthio, ethylthio, benzylthio, phenylthio and tolylthio groups; U1 and U2 are each a substituted or unsubstituted lower alkyl group or aryl group, provided that V1 and V2 may combine to form a 5- or 6-membered nitrogen containing heterocyclic ring (e.g., pyrazole ring, pyrrol ring, pyrrolidine ring, morpholine ring, pyperizine ring, pyridine, pyrimidine ring, etc.). Two methylene groups which are adjacent or distant by one may combine to form a 5- or 6-membered ring.
In cases where the compound represented by formula (S1), (S1xe2x80x941), (S2-1), (S3) or (S4) is substituted with a cationic- or anionic-charged group, a counter ion is formed by an anionic or cationic equivalent to compensate an intramolecular charge. As an ion necessary to compensate the intramolecular charge which is represented by X1, X11, X21, or X31, examples of cations include a proton, an organic ammonium ion (e.g., triethylammonium, triethanolammonium) and inorganic cations (e.g., cations of lithium, sodium and potassium); and examples of acid anions include halide ions (e.g., chloride ion, bromide ion, iodide ion), p-toluenesulfonate ion, perchlorate ion, tetrafluoroborate ion, sulfate ion, methylsulfate ion, ethylsulfate ion, methanesulfonate ion, trifluoromethanesulfonate ion).
The infrared sensitizing dye according to the invention is preferably a dye characterized in that a three ring-condensed heterocyclic nucleus is formed by bonding between a nitrogen atom contained in a benzothiazole ring and a carbon atom at a peri-position; or that the dye is a long chain polymethine dye, in which a sulfonyl group is substituted on the benzene ring of the benzothiazole ring.
Exemplary examples of the sensitizing dyes represented by formulas (S1), (S1-1), (S2-1), (S3) and (S4)are shown below, but are not limited to these. 
The infrared sensitizing dyes and spectral sensitizing dyes described above can be readily synthesized according to the methods described in F. M. Hammer, The Chemistry of Heterocyclic Compounds vol.18, xe2x80x9cThe cyanine Dyes and Related Compoundsxe2x80x9d (A. Weissberger ed. Interscience Corp., New York, 1964).
These sensitizing dyes may be used alone or in combination thereof. The combined use of sensitizing dyes is often employed for the purpose of supersensitization. A super-sensitizing compound, such as a dye which does not exhibit spectral sensitization or substance which does not substantially absorb visible light may be incorporated, in combination with a sensitizing dye, into the emulsion.
In cases when being super-sensitized, and specifically when a reducing agent is not deactivated, photosensitivity is enhanced, print-out is easily promoted after development. In such a case, the present invention is effective. In cases when being infrared-sensitized, an infrared sensitizing dye has an oxidation-reduction potential at which a silver halide or an organic silver salt is slightly reducible, easily producing a silver cluster forming fog silver in the presence of the reducing agent, even when placed in a dark room. The produced silver cluster also induces fogging as a catalyst nucleus, deteriorating storage stability in the dark room or promoting print-out when placed in a daylight room after development. Further, sensitivity of the infrared sensitive material extends to the thermal radiation region outside the visible region so that the present invention is effective for inhibiting print-out silver produced by thermal radiation. Such a effect is marked in infrared-sensitized photosensitive materials which is sensitized with a supersensitizer. Useful sensitizing dyes, dye combinations exhibiting super-sensitization and materials exhibiting supersensitization are described in RD17643 (published in December, 1978), IV-J at page 23, JP-B 9-25500 and 43-4933 (herein, the term, JP-B means published Japanese Patent) and JP-A 59-19032, 59-192242 and 5-341432.
In the invention, an aromatic heterocyclic mercapto compound represented by the following formula (6) is preferred as a supersensitizer:
Formula (6)
Ar-SM 
wherein M is a hydrogen atom or an alkali metal atom; Ar is an aromatic ring or condensed aromatic ring containing a nitrogen atom, oxygen atom, sulfur atom, selenium atom or tellurium atom. Such aromatic heterocyclic rings are preferably benzimidazole, naphthoimidazole, benzothiazole, naphthothiazole, benzoxazole, naphthooxazole, benzoselenazole, benzotellurazole, imidazole, oxazole, pyrazole, triazole, triazines, pyrimidine, pyridazine, pyrazine, pyridine, purine, and quinoline. Other aromatic heterocyclic rings may also be included.
A disulfide compound which is capable of forming a mercapto compound when incorporated into a dispersion of an organic silver salt and/or a silver halide grain emulsion is also included in the invention. In particular, a preferred example thereof is a disulfide compound represented by the following formula:
Formula (7)
Ar-S-S-Ar 
wherein Ar is the same as defined in formula (6). The aromatic heterocyclic rings described above may be substituted with a halogen atom (e.g., Cl, Br, I), a hydroxy group, an amino group, a carboxy group, an alkyl group (having one or more carbon atoms, and preferably 1 to 4 carbon atoms) or an alkoxy group (having one or more carbon atoms, and preferably 1 to 4 carbon atoms).
Exemplary examples of mercapto-substituted aromatic heterocyclic compound are shown below but are not limited to these.
The supersensitizer compound usable in the invention is incorporated into an emulsion layer containing the organic silver salt and silver halide grains, preferably in an amount of 0.001 to 1.0 mol, and more preferably 0.01 to 0.5 mo per mol of silver.
Binders suitable for the photothermographic materials used in the present invention are transparent or translucent, and generally colorless. Binders are natural polymers, synthetic resins, and polymers and copolymers, other film forming media; for example, gelatin, gum arabic, poly(vinyl alcohol), hydroxyethyl cellulose, cellulose acetate, cellulose acetatebutylate, poly(vinylpyrrolidone), casein, starch, poly(acrylic acid), poly(methylmethacrylic acid), poly(vinyl chloride), poly(methacrylic acid), copoly(styrene-maleic acid anhydride), copoly(styrene-acrylonitrile), copoly(styrene-butadiene), poly(vinyl acetal) series (for example, poly(vinyl formal)and poly(vinyl butyral), poly(ester) series, poly(urethane) series, phenoxy resins, poly(vinylidene chloride), poly(epoxide) series, poly(carbonate) series, poly(vinyl acetate) series, cellulose esters, poly(amide) series. These may be hydrophilic or hydrophobic polymers. Of these, as a binder preferable for the thermally developable light sensitive layer is polyvinyl acetals and more preferably polyvinyl butyral. Cellulose esters exhibiting higher softening temperature, such as triacetyl cellulose or cellulose acetatebutylate are preferred for non-light sensitive layers such as an over-coat layer or sub-coat layer, specifically, a protective layer or backing layer. These binders may be used in combination. The binder is used with a range effective to function as a binder. The effective range can optimally be determined by one skilled in the art. As a measure to hold at least an organic silver salt, a ratio of a binder to an organic silver salt, based on weight is preferably within a range of 15:1 to 1:2, more preferably 8:1 to 1:1. The amount of a binder in a light sensitive layer is preferably 1.5 to 6 g/m2, and more preferably 1.7 to 5 g/m2. The amount of less than 1.5 g/m2 results in an increase density of an unexposed area to levels unacceptable to practical use.
Inclusion of a cross-linking agent is specifically effective in the invention. Although the mechanism has not been elucidated, it was proved that the combined use of the cross-linking agent and the labile species-generating compound used relating to the invention gave advantageous effects on storage stability on the dark room and production of print-out silver under daylight. Although it is commonly known that the use of a cross-linking agent in such a binder as described above improves layer adhesion and lessens unevenness in development, it is unexpected that the use of the crosslinking agent in combination with the labile species-generating compound was effective in fog inhibition during storage and prevention of print-out after development.
Crosslinking agents usable in the invention include various commonly known crosslinking agents used for photographic materials, such as aldehyde type, epoxy type, vinylsulfon type, sulfonester type, acryloyl type, carbodiimide type crosslinking agents, as described in JP-A 50-96216.
Specifically preferred are an isocyanate type compound, epoxy compound and acid anhydride, as shown below. One of the preferred crosslinking agents is an isocyanate or thioisicyanate compound represented by the following formula:
Formula (8)
Xxe2x95x90Cxe2x95x90N-L-(Nxe2x95x90Cxe2x95x90X)v 
wherein v is 1 or 2; L is a bivalent linkage group of an alkylene, alkenylene, arylene or alkylarylene group; and X is an oxygen atom or a sulfur atom. An arylene ring of the arylene group may be substituted. Preferred substituent groups include a halogen atom (e.g., bromine atom, chlorine atom), hydroxy, amino, carboxy, alkyl and alkoxy.
The isocyanate crosslinking agent is an isocyanate compound containing at least two isocyanate group and its adduct. Examples thereof include aliphatic isocyanates, alicyclic isocyanates, benzeneisocyanates, naphthalenediisocyanates, biphenyldiisocyanates, diphenylmethandiisocyanates, triphenylmethanediisocyanates, triisocyanates, tetraisocyanates, their adducts and adducts of these isocyanates and bivalent or trivalent polyhydric alcohols. Exemplary examples are isocyanate compounds described in JP-A 56-5535 at pages 10-12, including: ethanediisocyanate, butanediisocyanate, hexanediisocyanate, 2,2-dimetylpentanediisocyanate, 2,2,4-trimethylpentanediisocyanate, decanediisocyanate, xcfx89,xcfx89xe2x80x2-diisocyanate-1,3-dimethylbenzol, xcfx89,xcfx89xe2x80x2-diisocyanate-1,2-dimethylcyclohexanediisocyanate, xcfx89,xcfx89xe2x80x2-diisocyanate-1,4-diethylbenzol, xcfx89,xcfx89xe2x80x2-diisocyanate-1,5-dimethylnaphthalene, xcfx89,xcfx89xe2x80x2-diisocyanate-n-propypbiphenyl, 1,3-phenylenediisocyanate, 1-methylbenzol-2,4-diisocyanate, 1,3-dimethylbenzol-2,6-diisocyanate, naphthalene-1,4-diisocyanate, 1,1xe2x80x2-naphthyl-2,2xe2x80x2-diisocyanate, biphenyl-2,4xe2x80x2-diisocyanate, 3,3xe2x80x2-dimethylbiphenyl-4,4xe2x80x2-diisocyanate, diphenylmethane-4,4xe2x80x2-diisocyanate, 2,2xe2x80x2-dimethyldiphenylmethane-4,4xe2x80x2-diisocyanate, 3,3xe2x80x2-dimethoxydiphenylmethane-4,4xe2x80x2-diisocyanate, 4,4xe2x80x2-diethoxydiphenylmethane-4,4xe2x80x2-diisocyanate, 1-methylbenzol-2,4,6-triisocyanate, 1,3,5-trimethylbenzene-2,4,6-triisocyanate, diphenylmethane-2,4,4xe2x80x2-triisocyanate, triphenylmethane-4,4xe2x80x2,4xe2x80x2-triisocyanate, tolylenediisocyanate, 1,5-naphthylenediisocyanate; dimmer or trimer adducts of these isocyanate compounds (e.g., adduct of 2-mole hexamethylenediisocyanate, adduct of 3 mole hexamethylenediisicyanate, adduct of 2 mole 2,4-tolylenediisocyanate, adduct of 3 mole 2,4-tolylenediisocyanate); adducts of two different isocyanates selected from these isocyanate compounds described above; and adducts of these isocyanate compounds and bivalent or trivalent polyhydric alcohol (preferably having upto 20 carbon atoms, such as ethylene glycol, propylene glycol, pinacol, and trimethylol propane), such as adduct of tolylenediisocyanate and trimethylolpropane, or adduct of hexamethylenediisocyanate and trimethylolpropane. Of these, adduct of isocyanate and polyhydric alcohol improves adhesion between layers, exhibiting high capability of preventing layer peeling, image slippage or production of bubbles. These polyisocyanate compounds may be incorporated into any portion of the photothermographic material, for example, into the interior of a support (e.g., into size of a paper support) or any layer on the light sensitive layer-side of the support, such as a light sensitive layer, surface protective layer, interlayer, antihalation layer or sublayer. Thus it may be incorporated into one or plurality of these layers.
The thiosocyanate type crosslinking agent usable in the invention is a compound having a thioisocyanate structure, corresponding to the isocyanates described above.
The crosslinking agents described above are used preferably in an amount of 0.001 to 2 mol, and more preferably 0.005 to 0.5 mol of silver.
The isocyanate compounds and thioisocyanate compounds used in the invention are preferably those which are capable of functioning as a hardener. Even when xe2x80x9cvxe2x80x9d of formula (8) is zero, i.e., even a compound containing only one functional group provides favorred effects. Exemplary examples thereof are shown below but are not limited to these. 
Further, preferred cross-linking agent is an epoxy compound containing at least an epoxy group and represented by the formula described below, or an acid anhydride.
The epoxy compound usable in the invention may be any one containing at least one epoxy group and is not limited with respect to the number of the epoxy group, molecular weight and other parameters. The epoxy group is preferably contained in the form of a glycidyl group through an ether bond or an imino bond in the molecule. The epoxy compound may be any one of a monomer, oligomer and polymer, in which the number of the epoxy group in the molecule is preferably 1 to 10 and more preferably 2 to 4. In cases where the epoxy compound is a polymer, it may be either one of a homopolymer and a copolymer. The number-averaged molecular weight (Mn) thereof is preferably 2,000 to 20,000. The epoxy compound used in the invention is preferably a compound represented by the following formula (9): 
wherein an alkylene group or arylene group represented by R in formula (9) may be substituted by a substituent selected from a halogen atom, a hydroxyalkyl group and an amino group; R in formula (9) preferably contains an amido-linkage, ether linkage or thioether linkage; a bivalent linkage group represented by X is preferably xe2x80x94SO2xe2x80x94, xe2x80x94SO2NHxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94Oxe2x80x94 or xe2x80x94NRxe2x80x2xe2x80x94, in which Rxe2x80x2 is a univalent linkage group and preferably an electron-withdrawing group.
Exemplary examples of the epoxy compound are shown below but are not limited to these. 
The epoxy compound may be used alone or combination thereof. The amount to be added is not specifically limited, but preferably 1xc3x9710xe2x88x926 to 1xc3x9710xe2x88x922 mol/m2, and more preferably 1xc3x9710xe2x88x925 to 1xc3x9710xe2x88x923 mol/m2. The epoxy compound may be added to any layer of a light sensitive layer, surface protective layer, interlayer, anti-halation layer and subbing layer provided on the light sensitive layer-side of the support and may be added to one or plurality of these layers. Further, it may be added to a layer provided on the opposite side of the support, in combination with the light sensitive layer-side. In the case of a photothermographic material having light sensitive layers on both sides of the support, it may be added to any one of the layers.
The acid anhydride used in the invention is preferably a compound containing at least an acid anhydride group represented as below: 
The acid anhydride usable in the invention may be any compound containing one or more acid anhydride group, the number of the acid anhydride group, molecular weight or other parameters are not specifically limited, and a compound represented by the following formula (B) is preferred: 
wherein Z is an atomic group necessary to form a monocyclic or polycyclic ring, which may be substituted. Examples of substituent include an alkyl group (e.g., methyl, ethyl, hexyl), an alkoxyl group (e.g., methoxy, ethoxy, octyloxy), an aryl group (e.g., phenyl, naphthyl, tolyl), hydroxy group, an aryloxy group (e.g., phenoxy), an alkylthio group (e.g., methylthio, butylthio), an arylthio group (e.g., phenylthio), an acyl group (e.g., acetyl, propionyl, butylyl), a sulfonyl group (e.g., methylsulfonyl, phenylsulfonyl), an acylamino group, a sulfonylamino group, an acyloxy group (e.g., acetoxy, benzoxy), carboxy group, cyano group, sulfo group and an amino group. It is preferred not to contain a halogen atom as a substituent.
Exemplary examples of the acid anhydride compound are shown below but are not limited to these. 
The acid anhydride compound may be used alone or combination thereof. The amount to be added is not specifically limited, but preferably 1xc3x9710xe2x88x926 to 1xc3x9710xe2x88x921 mol/m2, and more preferably 1xc3x9710xe2x88x924 to 1xc3x9710xe2x88x922 mol/m2. The acid anhydride compound may be added to any layer of a light sensitive layer, surface protective layer, interlayer, antihalation layer and subbing layer provided on the light sensitive layer-side of the support and may be added to one or plurality of these layers. Further, it may be added to a layer provided on the opposite side of the support, in combination with the light sensitive layer-side. In the case of a photothermographic material having light sensitive layers on both sides of the support, it may be added to any one of the layers.
Organic silver salts used in the invention are reducible silver source, and silver salts of organic acids or organic heteroacids are preferred and silver salts of long chain fatty acid (preferably having 10 to 30 carbon atom and more preferably 15 to 25 carbon atoms) or nitrogen containing heterocyclic compounds are more preferred. Specifically, organic or inorganic complexes, ligand of which have a total stability constant to a silver ion of 4.0 to 10.0 are preferred. Exemplary preferred complex salts are described in RD17029 and RD29963, including organic acid salts (for example, salts of gallic acid, oxalic acid, behenic acid, stearic acid, palmitic acid, lauric acid, etc.); carboxyalkylthiourea salts (for example, 1-(3-carboxypropyl)thiourea, 1-(3-caroxypropyl)-3,3-dimethylthiourea, etc.); silver complexes of polymer reaction products of aldehyde with hydroxy-substituted aromatic carboxylic acid (for example, aldehydes (formaldehyde, acetaldehyde, butylaldehyde, etc.), hydroxy-substituted acids (for example, salicylic acid, benzoic acid, 3,5-dihydroxybenzoic acid, 5,5-thiodisalicylic acid, silver salts or complexes of thiones (for example, 3-(2-carboxyethyl)-4-hydroxymethyl-4-(thiazoline-2-thione and 3-carboxymethyl-4-thiazoline-2-thione), complexes of silver with nitrogen acid selected from imidazole, pyrazole, urazole, 1,2,4-thiazole, and 1H-tetrazole, 3-amino-5-benzylthio-1,2,4-triazole and benztriazole or salts thereof; silver salts of saccharin, 5-chlorosalicylaldoxime, etc.; and silver salts of mercaptides. Of these organic silver salts, silver salts of fatty acids are preferred, and silver salts of behenic acid, arachidic acid and stearic acid are specifically preferred.
The organic silver salt compound can be obtained by mixing an aqueous-soluble silver compound with a compound capable of forming a complex. Normal precipitation, reverse precipitation, double jet precipitation and controlled double jet precipitation described in JP-A 9-127643 are preferably employed. For example, to an organic acid is added an alkali metal hydroxide (e.g., sodium hydroxide, potassium hydroxide, etc.) to form an alkali metal salt soap of the organic acid (e.g., sodium behenate, sodium arachidate, etc.), thereafter, the soap and silver nitrate are mixed by the controlled double jet method to form organic silver salt crystals. In this case, silver halide grains may be concurrently present.
In the present invention, organic silver salts have an average grain diameter of 10 xcexcm or less and are monodisperse. The average diameter of the organic silver salt as described herein is, when the grain of the organic salt is, for example, a spherical, cylindrical, or tabular grain, a diameter of the sphere having the same volume as each of these grains. The average grain diameter is preferably between 0.05 and 10 xcexcm, more preferably between 0.05 and 5 xcexcm and still more preferably between 0.05 and 0.5 xcexcm. Furthermore, the organic silver salt is preferably comprised of tabular grains.
Further, organic silver salts relating to this invention will be detailed with respect to preferred conditions. Thus, organic silver salts preferably comprise tabular grains which are preferably tabular grains exhibiting an aspect ratio of not less than 3, and to make smaller anisotropy in shape of two parallel opposite faces having a maximum area (also denoted as major faces) to achieve closer packing in the light sensitive layer, the tabular grains exhibit an average value of a needle ratio of not less than 1.1 and less than 10.0, and preferably not less than 1.1 and less than 5.0, which can be measured from the direction of the major face.
In this invention, the expression xe2x80x9ccomprise tabular grains exhibiting an aspect ratio of not less than 3xe2x80x9d means that the tabular grains account for at least 50% by number of the total organic silver salt grains. It is more preferred that the organic silver salt comprises tabular grains accounting for at least 60% by number of the total organic silver salt grains, still more preferably at least 70% and most preferably at least 80% by number.
The tabular organic silver salt grain having an aspect ratio of not less than 3 refers to an organic salt grain exhibiting a ratio of grain diameter to grain thickness, being a so-called aspect ratio (also denoted as AR) of 3 or more, which is defined below:
AR=diameter (xcexcm)/thickness (xcexcm) 
wherein when an organic silver salt particle is approximated to be a rectangular parallelepiped, the diameter is the maximum edge length (also denoted as MX LNG) and the thickness is the minimum edge length (also denoted as MN LNG).
The aspect ratio of the tabular organic silver salt particles is preferably within the range of 3 to 20, and more preferably 3 to 10. In the case of an aspect ratio of less than 3, the organic salt particles easily form closest packing and in the case of the aspect ratio being excessively high, organic silver salt particles are easily superposed and dispersed in the coating layer in the form of being brought into contact with each other, easily causing light scattering and leading to deterioration in transparency of the photothermographic material.
The grain diameter was determined in the following manner. An organic silver salt dispersion was diluted, dispersed on the grid provided with a carbon support membrane, and then photographed at a direct magnification of 5,000 times using a transmission type electron microscope (TEM, 2000 FX type, available from Nihon Denshi Co., Ltd.). The thus obtained negative electronmicrograph images were read as a digital image by a scanner to determine the diameter (circular equivalent diameter) using appropriate software. At least 300 grains were measured to determine the average diameter.
The grain thickness was determined using a transmission type electron microscope in the following manner. First, a light sensitive layer, coated onto a support, is pasted onto a suitable holder employing an adhesive and is cut perpendicular to the support surface employing a diamond knife to prepare an ultra-thin slice, at a thickness of 0.1 to 0.2 xcexcm. The thus prepared ultra-thin slice is supported on a copper mesh, and is placed onto a carbon membrane, which has been made to be hydrophilic by means of a glow discharge. Then, while cooling the resulting slice to not more than xe2x88x92130xc2x0 C., the image in a bright visual field is observed at a magnification of 5,000 to 40,000 employing a transmission electron microscope (hereinafter referred to as TEM), and then images are quickly recorded employing an image plate, a CCD camera, etc. In such a case, it is recommended to suitably select a portion of said slice, which has neither been torn nor distorted in the visual field for observation.
The carbon membrane, which is supported by an organic film such as an extremely thin collodion, Formvar, etc., is preferably employed, and a film composed of only carbon, which is obtained by forming the film on a rock salt substrate and then dissolving away the substrate or by removing the foregoing organic film, employing an organic solvent or ion etching, is more preferably employed. The acceleration voltage of said TEM is preferably 80 to 400 kV, and is most preferably 80 to 200 kV.
Details of other means such as electron microscopic technology and sample preparation techniques can be referred to in xe2x80x9cIgaku-Seibutsugaku Denshikenbikyo Kansatsuho (Medical and Biological Electron Microscopyxe2x80x9d, edited by Nippon Denshikenbikyo Gakkai, Kanto Shibu, (Maruzen), and xe2x80x9cDenshikenbikyo Seibutsu Shiryo Sakuseiho (Preparation Method of Biological Samples for Electron Microscopy)xe2x80x9d, edited by Nippon Denshikenbikyo Gakkai, Kanto Shibu, (Maruzen).
The TEM image, recorded in an appropriate medium, is decomposed to at least 1024xc3x971024 pixels or preferably at least 2048xc3x972048 pixels, and is then subjected to image processing employing a computer. In order to carry out image processing, an analogue image recorded on a film strip is converted into a digital image employing a scanner etc., and the resulting image is preferably subjected to shading correction, contrast-edge enhancement, etc., based on specific requirements. Thereafter, a histogram is prepared and the portions corresponding to organic silver are extracted employing binary processing. At least 300 grains of the organic silver salt were manually measured with respect to the thus extracted thickness employing appropriate software.
The average of the needle ratio of the tabular organic silver salt grains is determined according to the procedures described below.
First, a light sensitive layer, comprising tabular organic silver salt grains, is allowed to swell by employing an organic solvent which is capable of dissolving the binder of said light sensitive layer, and said layer is then peeled from the support. The operation is repeated five times, in which the peeled layer is subjected to ultrasonic cleaning with the above-mentioned solvent, and centrifugal separation, and the supernatant is removed. Further, the above-mentioned process is carried out under a photographic safelight.
Subsequently, dilution is carried out employing MEK (methyl ethyl ketone) so that the concentration of the organic silver solid portion becomes 0.01 percent. After carrying out ultrasonic dispersion, the resulting is dropped onto a polyethylene terephthalate film which has been made to be hydrophilic employing a glow discharge, and is subsequently dried.
The film, on which said grains are placed, is subjected to oblique evaporation of 3 nm thickness Pt-C by an electron beam from a 30xc2x0 angle to the film surface employing a vacuum evaporation unit, and thereafter, is preferably employed for observation.
The prepared sample is observed through a secondary electron image, obtained by employing a field emission scanning electron microscope (hereinafter referred to as PE-SEM) under a magnification of 5,000 to 20,000 at an acceleration voltage of 2 to 4 kV, and the resulting image is stored on suitable recording media.
For the above-mentioned processing, it is convenient to use a device which is capable of directly recording the memory data as digital information, which is obtained by AD converting image signals from the electron microscope body. However, analogue images recorded onto Polaroid film etc. can be converted to digital images employing a scanner etc., and the resulting images may be employed upon carrying out shading correction, contrast enhancement as well as edge enhancement, etc. if desired.
One image recorded in a suitable medium is decomposed to at least 1024xc3x971024 pixels and is preferably decomposed to 2048xc3x972048 pixels. Said decomposed image is preferably subjected to image processing employing a computer.
Procedures of the above-mentioned image processing are as follows. First, a histogram is prepared and portions corresponding to tabular organic silver salt grains having an aspect ratio of 3 or more are extracted employing binary processing. Inevitable coagulated grains are cut employing a suitable algorithm or a manual operation and are subjected to boarder extract. Thereafter, both maximum length (MX LNG) and minimum width (WIDTH) between two parallel lines are measured for at least 1000 grains, and the needle ratio of each grain is obtained employing the formula described below. The maximum length (MX LNG) is the maximum value of the straight length between two points within a grain. The minimum width between two parallel lines is a minimum distance of two parallel lines drawn circumscribing the grain.
Needle ratio=(MX LNG)/(WIDTH) 
Thereafter, the number average of the needle ratio is calculated for all measured particles. When measurements are carried out employing the above-mentioned procedures, it is desirable that in advance, employing a standard sample, the length correction (scale correction) per pixel as well as two-dimensional distortion correction of the measurement system is sufficiently carried out. As the standard sample, Uniform Latex Particles (DULP) marketed by Dow Chemical Co. in the USA are suitable. Polystyrene particles having a variation coefficient of less than 10 percent for a diameter of 0.1 to 0.3 xcexcm are preferred. Specifically, a type having a particle diameter of 0.212 xcexcm as well as a standard deviation of 0.0029 xcexcm is commercially available.
Details of image processing technology may be had by referring to xe2x80x9cGazoshori Oyogijutsu (Applied Technology in Image Processing)xe2x80x9d, edited by Hiroshi Tanaka, (Kogyo Chosa Kai). Image processing programs or apparatuses are not particularly restricted, as long as the above-mentioned operation is possible. Cited as one example is Luzex-III, manufactured by Nireko Co.
Methods to prepare organic silver salt grains having the above-mentioned shape are not particularly restricted. The optimization of various conditions such as maintaining the mixing state during the formation of an organic acid alkali metal salt soap and/or the mixing state during the addition of silver nitrate to said soap. After tabular organic silver salt grains employed in the present invention are preliminarily dispersed together with binders, surface active agents, etc., if desired, the resulting mixture is preferably dispersed and pulverized by a media homogenizer, a high pressure homogenizer, or the like. During said preliminary dispersion, ordinary stirrers such as an anchor type, a propeller type, etc., a high speed rotation centrifugal radial type stirrer (Dissolver), as a high speed shearing stirrer (homomixer) may be employed.
Furthermore, employed as said media homogenizers may be rolling mills such as a ball mill, a satellite ball mill, a vibrating ball mill, medium agitation mills such as a bead mill, atriter, and others such as a basket mill. Employed as high pressure homogenizers may be various types such as a type in which collision occurs against a wall or a plug, a type in which liquid is divided into a plurality of portions and said portions are subjected to collision with each other, a type in which liquid is forced to pass through a narrow orifice, etc. Examples of ceramics employed as the ceramic beads include Al1O3, BaTiO3, SrTiO3, MgO, ZrO, BeO, Cr2O3, SiO3, SiO2xe2x80x94Al2O3, Cr2O3xe2x80x94MgO, MgOxe2x80x94CaO, MoOxe2x80x94C, MgOxe2x80x94Al2O3 (spinel), SiC, TiO2, K2O, Na2O, BaO, PbO, B2O3, BeAl2O4, Y3Al5O12, ZrO2xe2x80x94Y2O3 (cubic zirconia), 3BeOxe2x80x94Al2O3-6SiO2 (artificial emerald), C (artificial diamond), SiO2-nH2O, silicone nitride, yttrium-stabilized-zirconia, zirconia-reinforced-alumina. Yttrium-stabilized-zirconia and zirconia-reinforced-alumina are preferably employed in view that little impurity is generated by friction among the beads or the classifier during classifying them. The ceramics containing zirconia are called zirconia as an abbreviation.
In devices employed for dispersing the tabular organic silver salt grains employed in the present invention, preferably employed as the members which are in contact with the organic silver salt grains are ceramics such as zirconia, alumina, silicone nitride, boron nitride, or diamond. Of these, zirconia is the one most preferably employed.
While carrying out of the above-mentioned dispersion, the binder is preferably added so as to achieve a concentration of 0.1 to 10 wt % with reference to the weight of the organic silver salt, and the temperature is preferably maintained at no less than 45xc2x0 C. from the preliminary dispersion to the main dispersion process. An example of the preferable operation conditions of a homogenizer, when employing high-pressure homogenizer as the dispersing machine, is twice or more operations at 300 to 1,000 kgf/cm2. In the case when a media-dispersing machine is employed, a circumferential speed of 6 to 13 m/sec. is preferable. In the case when zirconia is employed as a part of the beads or of the machine, it is ground and mixed into the dispersion during the mixing process. This is specifically advantageous in view of photographic characteristics. Fragments of zirconia may be supplementally added to the dispersion or preliminarily added during preliminary dispersing. A high concentration zirconia liquid can be obtained, for example, by circulating methylethylketone in a bead mill filled with zirconia beads. The obtained zirconia liquid may be added in the adequate amount at adequate stages. The content of the zirconia in a light sensitive emulsion containing light sensitive silver halide and anorganic silver salt is preferably 0.01 to 0.5 mg, and more preferably 0.01 to 0.3 mg per g of silver. The zirconia is preferably in the form of fine particles having a diameter of not more than 0.02 xcexcm.
In one preferred embodiment of this invention, light sensitive silver halide used in this invention is subjected to chemical sensitization which is performed using an organic sensitizer containing a chalcogen atom in the absence of an oxidizing agent during the manufacturing process of the photothermographic material, the silver halide being mixed with the organic silver salt, dispersed, dewatered and dried. One feature of the light sensitive emulsion used in the invention is that when the cross section, vertical to the support of the photothermographic material is observed through an electron microscope, organic silver salt particles exhibiting a grain projected area of less than 0.025 xcexcm2 account for at least 70% of the total grain projected area and organic silver salt particles exhibiting a grain projected area of not less than 0.2 xcexcm2 account for not more than 10% of the total grain projected area. In such a case, coagulation of the organic silver salt grains is minimized in the light sensitive emulsion, resulting in a homogeneous distribution thereof.
The conditions for preparing the light sensitive emulsion having such a feature are not specifically limited but include, for example, mixing at the time of forming an alkali metal soap of an organic acid and/or mixing at the time of adding silver nitrate to the soap being maintained in a favorable state, optimization of the ratio of the soap to the silver nitrate, the use of a media dispersing machine or a high pressure homogenizer for dispersing pulverization, wherein dispersion is conducted preferably in a binder content of 0.1 to 10% by weight, based on the organic silver salt, the dispersion including the preliminary dispersion is carried out preferably at a temperature of not higher than 45xc2x0 C., and a dissolver, as a stirrer is preferably operated at a circumferential speed of at least 2.0 m/sec.
The projected area of organic silver salts grain having a specified projection area and the desired proportion thereof, based on the total grain projection area can be determined by the method using a transmission type electron microscope (TEM) in a similar manner, as described in the determination of the average thickness of tabular grains having an aspect ratio of 3 or more. In this case, coagulated grains are regarded as a single grain when determining the grain area (AREA). At least 1000 grains, and preferably at least 2000 grains are measured to determine the area and classified into three groups, i.e., A: less than 0.025 xcexcm2, B: not less than 0.025 xcexcm2 and less than 0.2 xcexcm2 and C: more than 0.2 xcexcm2. In this invention, it is preferable that the total projected area of grains falling within the range of xe2x80x9cAxe2x80x9d accounts for at least 70% of the projected area of the total grains and the total projected area of grains falling within the range of xe2x80x9cCxe2x80x9d accounts for not more than 10% of the projected area of total grain.
The organic silver salt grains used in this invention are preferably monodisperse. The degree of monodispersion is preferably 1 to 30% and monodisperse particles in this range lead to the desired high density images. The degree of monodispersion is defined as below:
Degree of monodispersion=(standard deviation of particle size)/(average particle size)xc3x97100 (%). The average particle size of organic silver salt is preferably 0.01 to 0.8 xcexcm, and more preferably 0.05 to 0.5 xcexcm. The particle size refers to the diameter of a circle having an area equivalent to the projected area of the particle (i.e., circular equivalent diameter).
To prevent hazing of the light sensitive material, the total amount of silver halide and organic silver salt is preferably 0.5 to 2.2 g in equivalent converted to silver per m2, thereby leading to high contrast images.
The thermally developable photosensitive material (hereinafter, also referred to as photothermographic material), which forms images upon thermal development, comprises a reducible silver source (such as an organic silver salt), light sensitive silver halide, reducing agent and optionally an image toning agent to modify silver image color, which are dispersed in an (organic) binder matrix. The photothermographic material is stable at ordinary temperatures, which is developed, after exposure, upon heating at a high temperature (e.g., 80 to 140xc2x0 C.). On heating, silver is formed through oxidation-reduction reaction between the organic silver salt (which acts as an oxidant) and the reducing agent. The oxidation-reduction reaction is catalyzed by silver latent images formed upon exposure to light. Silver formed by reaction of the organic silver salt in exposed areas provides a black image in contrast to non-exposed areas, forming images. This reaction process proceeds without supplying processing solution such as water from the exterior.
Image toning agents are preferably incorporated into the photothermographic material used in the present invention. Examples of preferred image toning agents are disclosed in Research Disclosure Item 17029, and include the following:
imides (for example, phthalimide), cyclic imides, pyrazoline-5-one, and quinazolinone (for example, succinimide, 3-phenyl-2-pyrazoline-5-on, 1-phenylurazole, quinazoline and 2,4-thiazolidione); naphthalimides (for example, N-hydroxy-1,8-naphthalimide); cobalt complexes (for example, cobalt hexaminetrifluoroacetate), mercaptans (for example, 3-mercapto-1,2,4-triazole); N-(aminomethyl)aryldicarboxyimides[for example, N-(dimethylaminomethyl)phthalimide]; blocked pyrazoles, isothiuronium derivatives and combinations of certain types of light-bleaching agents (for example, combination of N,Nxe2x80x2-hexamethylene(1-carbamoyl-3,5-dimethylpyrazole), 1,8-(3,6-dioxaoctane)bis-(isothiuroniumtrifluoroacetate), and 2-(tribromomethyl-sulfonyl)benzothiazole; merocyanine dyes (for example, 3-ethyl-5-((3-etyl-2-benzothiazolinylidene-(benzothiazolinylidene))-1-methylethylidene-2-thio-2,4-oxazolidinedione); phthalazinone, phthalazinone derivatives or metal salts thereof (for example, 4-(1-naphthyl)phthalazinone, 6-chlorophthalazinone, 5,7-dimethylphthalazinone, and 2,3-dihydro-1,4-phthalazinedione); combinations of phthalazinone and sulfinic acid derivatives (for example, 6-chlorophthalazinone and benzenesulfinic acid sodium, or 8-methylphthalazinone and p-trisulfonic acid sodium); combinations of phthalazine and phthalic acid; combinations of phthalazine (including phthalazine addition products) with at least one compound selected from maleic acid anhydride, and phthalic acid, 2,3-naphthalenedicarboxylic acid or o-phenylenic acid derivatives and anhydrides thereof (for example, phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid, and tetrachlorophthalic acid anhydride); quinazolinediones, benzoxazine, naphthoxazine derivatives, benzoxazine-2,4-diones (for example, 1,3-benzoxazine-2,4-dione); pyrimidines and asymmetry-triazines (for example, 2,4-dihydroxypyrimidine), and tetraazapentalene derivatives (for example, 3,6-dimercapto-1,4-diphenyl-1H,4H-2,3a,5,6a-tatraazapentalene). Preferred image color control agents include phthalazone or phthalazine.
Antifoggants may be incorporated into the photothermographic material to which the present invention is applied. The substance which is known as the most effective antifoggant is a mercury ion. The incorporation of mercury compounds as the antifoggant into photosensitive materials is disclosed, for example, in U.S. Pat. No. 3,589,903. However, mercury compounds are not environmentally preferred. As mercury-free antifoggants, preferred are those antifoggants as disclosed in U.S. Pat. Nos. 4,546,075 and 4,452,885, and P-A 59-57234. Particularly preferred mercury-free antifoggants are heterocyclic compounds having at least one substituent, represented by xe2x80x94C(X1)(X2)(X3) (wherein X1 and X2 each represent halogen, and X3 represents hydrogen or halogen), as disclosed in U.S. Pat. Nos. 3,874,946 and 4,756,999. As examples of suitable antifoggants, employed preferably are compounds described in paragraph numbers [0030] through [0036] of JP-A 9-288328. Further, as another examples of suitable antifoggants, employed preferably are compounds described in paragraph numbers [0062] and [0063] of JP-A 9-90550. Furthermore, other suitable antifoggants are disclosed in U.S. Pat. Nos. 5,028,523, and European Patent 600,587; 605,981 and 631,176.
In the present invention, a matting agent is preferably incorporated into the image forming layer side. In order to minimize the image abrasion after thermal development, the matting agent is provided on the surface of a photosensitive material and the matting agent is preferably incorporated in an amount of 0.5 to 30 per cent in weight ratio with respect to the total binder in the emulsion layer side.
In cases where a non light sensitive layer is provided on the opposite side of the support to the light sensitive layer, it is preferred to incorporate a matting agent into at least one of the non-photosensitive layer (and more preferably, into the surface layer) in an amount of 0.5 to 40% by weight, based on the total binder on the opposite side to the light sensitive layer.
Materials of the matting agents employed in the present invention may be either organic substances or inorganic substances. Examples of the inorganic substances include silica described in Swiss Patent No. 330,158, etc.; glass powder described in French Patent No. 1,296,995, etc.; and carbonates of alkali earth metals or cadmium, zinc, etc. described in U.K. Patent No. 1,173,181, etc. Examples of the organic substances include starch described in U.S. Pat. No. 2,322,037, etc.; starch derivatives described in Belgian Patent No. 625,451, U.K. Patent No. 981,198, etc.; polyvinyl alcohols described in Japanese Patent Publication No. 44-3643, etc.; polystyrenes or polymethacrylates described in Swiss Patent No. 330,158, etc.; polyacrylonitriles described in U.S. Pat. No. 3,079,257, etc.; and polycarbonates described in U.S. Pat. No. 3,022,169.
The shape of the matting agent may be crystalline or amorphous. However, a crystalline and spherical shape is preferably employed. The size of a matting agent is expressed in the diameter of a sphere having the same volume as the matting agent. The particle diameter of the matting agent in the present invention is referred to the diameter of a spherical converted volume. The matting agent employed in the present invention preferably has an average particle diameter of 0.5 to 10 xcexcm, and more preferably of 1.0 to 8.0 xcexcm. Furthermore, the variation coefficient of the size distribution is preferably not more than 50 percent, is more preferably not more than 40 percent, and is most preferably not more than 30 percent. The variation coefficient of the size distribution as described herein is a value represented by the formula described below:
(Standard deviation of particle diameter)/(average particle diameter)xc3x97100 
The matting agent according to the present invention can be incorporated into any layer. In order to accomplish the object of the present invention, the matting agent is preferably incorporated into the layer other than the light sensitive layer, and is more preferably incorporated into the farthest layer from the support.
Addition methods of the matting agent include those in which a matting agent is previously dispersed into a coating composition and is then coated, and prior to the completion of drying, a matting agent is sprayed. When plural matting agents are added, both methods may be employed in combination.
In addition to these materials, a variety of additives may be optionally incorporated into the light sensitive layer, non-photosensitive layer or other component layer(s). The photothermographic materials of the invention may be added with a surfactant, an antioxidant, a stabilizer, a plasticizer, a UV absorbent or a coating aid. As these additives and other additives described above are preferably employed compounds described in RD17029 (June, 1978, pages 9 to 15).
Supports usable in the photothermographic materials include various kinds of polymeric materials, glass, wool fabric, cotton fabric, paper, metal (e.g., aluminum) and those which are convertible to flexible sheets or rolls are preferred in terms of handling as information recording material. Preferred supports usable in photothermographic materials are plastic resin films (e.g., cellulose acetate film, polyester film, polyethylene terephthalate film, polyethylene naphthalate film, polyamide film, polyimide film, cellulose triacetate film, polycarbonate film) and biaxially stretched polyethylene terephthalate film is specifically preferred. The thickness of the support is preferably 50 to 300 xcexcm, and more preferably 70 to 180 xcexcm.
In the present invention, to improve an electrification property, a conducting compound such as a metal oxide and/or a conducting polymer can be incorporated into a construction layer. These compounds can be incorporated into any layer, preferably into a sublayer, a backing layer and an intermediate layer between a light sensitive layer and a sublayer, etc. In the present invention, the conducting compounds described in U.S. Pat. No. 5,244,773, column 14 through 20, are preferably used.
The coating method of the light sensitive layer, protective layer and backing layer is not specifically limited. Coating can be conducted by any method known in the art, including air knife, dip-coating, bar coating, curtain coating, and hopper coating. Two or more layers can be simultaneously coated. As a solvent for coating solution are employed organic solvents such as methyl ethyl ketone (also denoted as MEK), ethyl acetate and toluene.
The photothermographic material according to the invention comprises a support having thereon a light sensitive layer, and preferably further on the light sensitive layer having a non-photosensitive layer. For example, it is preferred that a protective layer is provided on the light sensitive layer to protect the light sensitive layer and that a back coating layer is provided on the opposite side of the support to the light sensitive layer to prevent adhesion between photosensitive materials or sticking of the photosensitive material to a roller. Further, there may be provided a filter layer on the same side or opposite side to the light sensitive layer to control the amount or wavelengths of light transmitting the thermally developable light sensitive layer. Alternatively, a dye or pigment may be incorporated into the light sensitive layer. In this case, dyes described in JP-A 8-201959 are preferably used therein. The light sensitive layer may be comprised of plural layers. To adjust contrast, a high-speed layer and low speed layer may be provided in combination. Various adjuvants may be incorporated into the light sensitive layer, non-photosensitive layer or other component layer(s).
The photothermographic material, which is stable at ordinary temperatures, is exposed and heated at a high temperature (preferably 80 to 200xc2x0 C., and more preferably 100 to 150xc2x0 C.) to undergo development. In cases when heated at a temperature of lower than 80xc2x0 C., sufficient image density can be obtained within a short time. Further, in cases when heated at a temperature of higher than 200xc2x0 C., a binder melts and is transferred to a roller, adversely affecting not only images but also transportability and a developing machine. The organic silver salt (functioning as an oxidant) and the reducing agent undergo oxidation-reduction reaction upon heating to form silver images. The reaction process proceeds without supplying any processing solution such as water.
Any light source within the infrared region is applicable to exposure of the photothermographic material and infrared semiconductor lasers (780 nm, 820 nm) are preferred in terms of high power and transmission capability through the photosensitive material.
In the invention, exposure is preferably conducted by laser scanning exposure. It is also preferred to use a laser exposure apparatus, in which scanning laser light is not exposed at an angle substantially vertical to the exposed surface of the photosensitive material. The expression xe2x80x9claser light is not exposed at an angle substantially vertical to the exposed surfacexe2x80x9d means that laser light is exposed preferably at an angle of 55 to 88xc2x0, more preferably 60 to 86xc2x0, still more preferably 65 to 84, and optimally 70 to 82xc2x0. When the photosensitive material is scanned with laser light, the beam spot diameter on the surface of the photosensitive material is preferably not more than 200 xcexcm, and more preferably not more than 100 xcexcm. Thus, the less spot diameter preferably reduces an angle displacing from verticality of the laser incident angle. The lower limit of the beam spot diameter is 10 xcexcm. The thus laser scanning exposure can reduce deterioration in image quality caused by to reflection light, such as occurrence of interference fringe-like unevenness.
Exposure applicable in the invention is conducted preferably using a laser scanning exposure apparatus producing longitudinally multiple scanning laser light, whereby deterioration in image quality such as occurrence of interference fringe-like unevenness is reduced, as compared to scanning laser light with longitudinally single mode. Longitudinal multiplication can be achieved by a technique of employing backing light with composing waves or a technique of high frequency overlapping. The expression xe2x80x9clongitudinally multiplexe2x80x9d means that the exposure wavelength is not a single wavelength. The exposure wavelength distribution is usually not less than 5 nm and not more than 10 nm. The upper limit of the exposure wavelength distribution is not specifically limited but usually about 60 nm.
It is preferred that when subjected to thermal development, the photothermographic material contains an organic solvent. Examples of solvents include ketones such as acetone, isophorone, ethyl amyl ketone, methyl ethyl ketone, methyl isobutyl ketone; alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, diacetone alcohol, cyclohexanol, and benzyl alcohol; glycols such as ethylene glycol, dimethylene glycol, triethylene glycol, propylene glycol and hexylene glycol; ether alcohols such as ethylene glycol monomethyl ether, and dimethylene glycol monomethyl ether; ethers such as ethyl ether, dioxane, and isopropyl ether; esters such as ethyl acetate, butyl acetate, amyl acetate, and isopropyl acetate; hydrocarbons such as n-pentane, n-hexane, n-heptane, cyclohexene, benzene, toluene, xylene; chlorinated compounds such as chloromethyl, chloromethylene, chloroform, and dichlorobenzene; amines such as monomethylamine, dimethylamine, triethanol amine, ethylenediamine, and triethylamine; and water, formaldehyde, dimethylformaldehyde, nitromethane, pyridine, toluidine, tetrahydrofuran and acetic acid. The solvents are not to be construed as limiting these examples. These solvents may be used alone or in combination.
The solvent content in the photosensitive material can be adjusted by varying conditions such as temperature conditions at the drying stage after the coating stage. The solvent content can be determined by means of gas chromatography under the conditions suitable for detecting the solvent. The total solvent content (based on weight) of the photothermographic material used in the invention is preferably adjusted to be 5 to 1,000 mg per m2 of the photothermographic material and more preferably 100 to 500 mg/m2 (based on the weight of constituting components of the photosensitive material, except for a support). The solvent content within the range described above leads to a photothermographic material with low fog density as well as high sensitivity.